NOVA

NARRATOR: March 26, 2024.

A heavily loaded container ship careens out of control and heads straight toward one of the supports of the Francis Scott Key Bridge in Baltimore.

OFFICER (on radio): C-13 Dispatch, the whole bridge just fell down.

The whole bridge just collapsed.

SEARCH UNIT MEMBER: Key Bridge is down.

Several vehicles in the water.

NARRATOR: It's a race to find survivors...

OFFICER 2: They're bringing a victim in now.

OFFICER 3: Okay.

(helicopter whirring) NARRATOR: ...and remove the 50,000 tons of debris that completely cut off this vital port.

ESTEE PINCHASIN: People are looking at us and saying, "What's the plan?"

NARRATOR: The scale of the salvage operation is daunting.

They need to reopen this crucial shipping route as quickly as possible.

ROBYN BIANCHI: You could instantly feel the ripple effects on our economy, on Baltimore.

PINCHASIN: We knew that it was gonna be very dangerous, very complex.

STEPHEN SKIPPEN: It will certainly make your hair stand up on the back of your neck seeing the steel drop as you're making cuts.

NARRATOR: Investigators scramble to uncover what went wrong, asking, how did a modern ship suffer such a catastrophic loss of control?

And why did a single point of impact cause the entire bridge to collapse?

The problem you have is that ships have gotten bigger and larger, and the infrastructure has not kept up with them.

NARRATOR: An estimated two-and-a-half thousand U.S. bridges remain at risk of ship strikes.

ABI AGHAYERE: This could have been prevented.

NARRATOR: As vessels grow in size, can our infrastructure handle it?

"Baltimore Bridge Collapse," right now on "NOVA."

NARRATOR: The port of Baltimore, Maryland, is one of the busiest ports on the East Coast, and its history is as long and storied as the United States itself.

So much of Baltimore's identity is actually wrapped up in its port.

The Port of Baltimore has been one of the earliest trade points for not just the state, but for this country.

I mean, that port is our economic heartbeat in the state of Maryland.

NARRATOR: Every year, it handles cargo worth more than $80 billion and supports more than 20,000 jobs.

The Port of Baltimore supplies the nation with critical goods from food to pharmaceuticals.

The port has 17 separate terminals for container operations, cruise ships, and bulk cargo.

2,000 vessels dock here every year.

It can handle some of the world's largest ships, each carrying more than 15,000 containers.

To reach the port terminals, all ships must pass under the Francis Scott Key Bridge, which carries four lanes of road traffic 1.7 miles across the bay.

The ships must negotiate a narrow shipping channel that runs between the main supports of the bridge.

MOORE: From a practical perspective, the Key Bridge gets you to the Port of Baltimore.

(chuckles): But it's so much more than that.

It's our skyline.

The backdrop for Baltimore.

NARRATOR: In the early hours of Tuesday, March 26, 2024, 12 ships are docked in the port.

At 12: 36 A.M., one sets sail for Sri Lanka with 21 crew and two local pilots onboard.

It's called Dali, a 124,000-ton, 947-foot-long ship.

It's part of a modern class of ships so massive that only certain ports have channels deep enough to allow them to pass.

The vessel is almost full, laden with nearly 4,700 containers, carrying everything from soybeans to perfume.

But as the ship approaches the bridge, there's a sudden loss of electrical power.

The ship is plunged into darkness.

At 1: 26 A.M., one of the pilots onboard sends out a distress call, which alerts the police.

DISPATCHER: Hold all traffic on the Key Bridge.

There's a ship approaching that has just lost their steering.

OFFICER: I'm holding traffic now.

I was driving, but we stopped prior to the bridge.

MOORE: We had workers on that bridge.

These are people who are fixing potholes.

They were fixing potholes while we slept.

DISPATCHER: Just make sure no one's on the bridge right now.

If there's a crew up there, you might want to notify whoever the foreman is, see if we can get them off the bridge temporarily.

OFFICER: Once you get here, I'll go grab the workers on the Key Bridge and then stop the outer loop.

NARRATOR: But the police are unable to reach the workers in time.

OFFICER: C-13 Dispatch, the whole bridge just fell down.

The whole bridge just collapsed.

MOORE: My chief of staff, he sent me a, a note saying, "I just sent you a video."

The video was of the bridge collapsing and it literally took my breath away.

Uh, it just didn't look real.

We knew that one of the most tragic moments in our state's history had just occurred.

BYSTANDER: That's the Key Bridge, Key Bridge.

OFFICER: Yeah, it is.

BYSTANDER: Wow!

NARRATOR: The container ship Dali has smashed into and destroyed the bridge.

Immediately, Coast Guard and police search teams race to the scene.

MAN: The whole bridge has collapsed.

And the bridge is sitting on top of the container ship.

DAVID O'CONNELL: We started searching almost instantaneously 'cause our station was very close.

If you just get us near where the other Fire Department assets are out there.

MAN: You got it, you got it.

SEARCH UNIT MEMBER: Key Bridge is down.

It was last reported, at least several vehicles in the water and several people still unaccounted for.

O'CONNELL (voiceover): It was night time, it was a fairly clear night though so they could see, they had visibility.

They had lights as well.

RESCUE OFFICER: Hey what's going on, man?

We've got one out of the water.

Okay.

O'CONNELL (voiceover): There was one survivor that went into the water in his vehicle and he was clinging to a piece of debris when he was picked up.

RESCUE OFFICER: They're bringing the victim in now.

Okay.

O'CONNELL (voiceover): Incredible that somebody would survive that fall and be rescued.

It gave hope to the rescuers that we were gonna find other victims as well that survived the fall.

SEARCH UNIT: 343 Sierra.

I have nothing at the moment.

NARRATOR: The search teams have rescued one construction worker, but six are still missing.

First light reveals the full scale of the disaster.

POLICE OFFICER (on radio): God.

POLICE OFFICER 2 (on radio): Natural one to Command.

NARRATOR: The Dali had so much momentum, it completely destroyed one of the main support piers that held up the Key Bridge.

The ship has run aground, and is trapped by the wreckage.

Almost 3,000 feet of the crossing have collapsed into the Patapsco River.

OFFICER (on radio): We've got some investigators onboard, we'll be in the area of the, uh, bridge.

NARRATOR: Fortunately, the Dali's crew is unscathed and the hull has not been breached.

But no ships can get in or out of the Port of Baltimore.

50,000 tons of debris completely block the main shipping channel; the port is shut down.

As the day ends, there's still no sign of the six missing workers.

MOORE (voiceover): I remember speaking with the person who was running the operation from the Coast Guard, and I said, "What is the probability that we're gonna find someone alive at this point?"

And the answer that he gave was "Governor, "my professional assessment is zero.

There's a zero percent chance."

I remember one of the family members who said "Quiero el cuerpo."

Which is "I, I want the body."

NARRATOR: While the search for lost workers continues, the city begins to grapple with the sheer scale of the problem.

PINCHASIN: Never seen anything of that magnitude before.

We had the bridge collapse with the vessel, everything mangled up on top and around.

You had these big, large spans that were just laying in the water.

You see four-inch steel that's been bent-- bent!

How can you even start to think about the force?

People are looking at us and saying, "What's the plan?"

NARRATOR: The stakes are high.

Every day the port is closed costs $15 million in lost revenue, and more than a thousand containers a day cannot move in or out.

There are fears the closure could impact the wider economy, affecting commerce across the country.

The U.S. Army Corps of Engineers and the U.S. Coast Guard bring in the Navy and three specialist salvage teams.

PINCHASIN: The number one priority was recovering the missing personnel.

The number two priority was opening the channel.

NARRATOR: The situation calls for a rapid response of unprecedented scale.

The team must free the Dali from the 5,000 ton bridge section that pins down its bow, so they can tow the vessel away.

But before they can fully reopen the port, they must also cut the collapsed sections of bridge into smaller pieces and lift them from the river.

50,000 tons of mangled steel and concrete.

It's a huge challenge.

BIANCHI: And you just think, how?

How are we going to?

Seeing it on the news, seeing it in photos, I was not prepared to see it, you know, up close and personal.

NARRATOR: Robyn Bianchi is part of a salvage team brought in to clear the bridge wreckage from the main shipping channel.

WORKER: This is gonna be a hell of a clean-up.

BIANCHI: And it was just so unfathomable, the amount of damage and wreckage and how massive everything was.

And we instantly knew, well, we can't make any plans on how to raise this wreckage out of the water until we know what it looks like under the water.

NARRATOR: The salvage team's first task is to send divers down into the mass of twisted steel.

They must try to make sense of how the mangled beams and smashed road are positioned on the riverbed.

It's a key step before they can make a plan to disassemble the tangled structure safely.

BIANCHI: They're entering a very inherently dangerous situation.

SALVAGE TEAM MEMBER: You have the hanger, go back to the hanger part.

(respirating) BIANCHI: Like, there was no visibility, so I kind of compared it to a metal jungle gym underwater.

Right, you have this rusty, metal jungle gym that was plopped into zero visibility.

You turn off all the lights in the room and try and tell me all the pieces of where they connect, of that metal jungle gym.

Oh, and by the way the jungle gym's completely twisted and it looks nothing like it did when it was built to perfection.

(respirating) SALVAGE TEAM MEMBER: How far from the first gusset that you just left?

DIVER: Maybe about six foot?

SALVAGE TEAM MEMBER: Okay.

BIANCHI: You want to really meticulously guide them through.

All right, when you have your hand on this section, we're gonna call this section J, so I'd have them mark it with a little, um, underwater marker and that correlated to section J on this specific engineering plan.

SALVAGE TEAM MEMBER: Can you see the other side of that pendant?

Is it disconnected?

DIVER: Yeah, it looks still connected to a beam, uh, like a truss.

BIANCHI: We were able to kind of map out what it looks like underwater.

You verify and validate with comms, and then you move on to the next objective.

SALVAGE TEAM MEMBER: So how much do you have exposed from the mudline to the top of the gusset plate?

DIVER: Standby, I'll grab the tape.

BIANCHI: It was very dangerous for divers because you'd turn left and you went underneath something, like a piece of rebar that's sticking out, you have to remember that that's where you have to come back out otherwise you could become trapped.

Everything that could make diving more difficult, it was on this job.

NARRATOR: Visibility is no more than one or two feet.

To give the divers a clearer picture, the team deploys underwater drones equipped with sonar to map the wreckage.

Together with the hands-on dive surveys, these scans will help the team decide where to cut and lift each section of debris.

SUAREZ: This volumetric sonar uses very precise location data from various scans to essentially stitch together multiple scans to create a 3D model.

The picture spoke a thousand words.

It just gave a really high fidelity view of exactly what everyone was dealing with on the bottom.

PINCHASIN: Down at the bottom is where you had a big, mangled mess, especially in the center of the channel.

Not just the steel, but the reinforced concrete from the road bed.

What we didn't realize until we were able to get some 3D imagery of just how catastrophic the collapse was.

NARRATOR: While the salvage teams prepare their work plans, the National Transportation Safety Board begins an investigation to answer the questions-- what caused the ship to lose power and collide with the bridge?

And why did the structure collapse so catastrophically?

Investigators board the Dali on the day of the accident.

They document the scene, and meticulously gather physical and electronic evidence.

They interview the captain and 20 crew, all of whom remain on the ship because of visa restrictions.

They retrieve the ship's voyage data recorder, which stores audio from the Dali's bridge and details engine and rudder commands.

Marcel Muise is the investigator in charge of the Dali accident.

We're looking at different aspects of the incident to answer, how do we stop ships from losing power?

Assuming that we can't stop ships from losing power, how do we stop them from hitting things?

Assuming we can't stop ships from hitting bridges, how do we stop them from falling down?

NARRATOR: To answer this question, the team must study the structure of the bridge to understand how it collapsed.

The critical structural element that allowed the Key Bridge to reach all the way across the river was its massive truss, a lattice of steel beams arranged in triangles that made it light yet strong.

Spanning 1,200 feet, it was the third longest continuous bridge truss ever constructed.

But at this extreme length, it would have buckled under its own weight without support.

(metal buckling) It needed two large reinforced concrete piers to hold it up, dividing it into three shorter spans.

The Key Bridge was a feat of structural engineering but it had a fundamental weakness.

AGHAYERE: If you take away a critical element, you lose that equilibrium and the structure is gonna collapse.

NARRATOR: With one of the main support piers destroyed, the bridge cannot span such a distance, and begins to break apart.

These two span becomes one span for which this bridge was not designed.

You can see separation happening at the bottom.

The collapse is progressing, actually.

And that part is gonna separate, and you can see it's breaking off because of that tension.

NARRATOR: Without the rest of the truss to hold it in tension, the remaining span becomes unbalanced and collapses under its own weight.

AGHAYERE: A typical progressive collapse, uh, scenario.

Let's take a test run on this bridge right now.

NARRATOR: When the Francis Scott Key Bridge was opened in 1977, a failure on this scale must have seemed impossible.

But back then, the largest container ships were less than half the size of the Dali.

So the main piers of the Key Bridge were built with concrete and timber fenders, designed to absorb the impact of ships at the time.

AGHAYERE: There's no way that they could have envisioned that it would be a 100,000 ton ship striking the bridge.

REPORTER (voiceover): So I've just completed the maiden voyage for NewsWatch 2 across the Francis Scott Key Bridge, and starting Wednesday at 10:00 A.M., you'll get your chance.

SAL MERCOGLIANO: We move more goods today than at any time in our history.

The problem you have is that ships have gotten bigger and larger, and the infrastructure has not kept up with them.

Lifespan for bridges is about 75 years.

Ships on the other hand can last anywhere from 20 to 30 years, and so two to three different generations of ships can happen during the lifespan of just one bridge.

Infrastructure can't change as fast as shipping can change.

AGHAYERE: The force from the Dali was huge.

It was a huge, huge force.

NARRATOR: The ship's enormous mass acts as a multiplier of its velocity to devastating effect.

AGHAYERE: There's truly no bridge that I know that would be designed for 34 million pounds of impact force.

So it's not surprising that it went down.

What is surprising is that there was no protection of those piers.

This could have been prevented.

NARRATOR: Five days after the collision, four construction workers are still missing.

11 ships are trapped in the port, many more unable to get into the port are diverted as far south as Brunswick, Georgia, over 700 miles away.

The sonar scans reveal that huge sections of the collapsed bridge are anchored deep in the mud on the riverbed.

This will make the task to remove the wreckage even tougher.

Engineers bring in 22 massive floating cranes, some urgently reassigned from other jobs 130 miles away.

These cranes can lift up to 500 tons.

But even that isn't enough.

BIANCHI: Some of those pieces were more than a thousand tons.

NARRATOR: They must risk destabilizing the tangled mess by cutting the massive truss sections into more manageable pieces.

BIANCHI: So we kind of had to figure out how many bites do we have to take out of this large structure that's underwater?

How do you eat an elephant one bite at a time?

NARRATOR: But each bent steel beam holds a potentially lethal surprise for the workers about to cut through them.

They're loaded with enormous stored energy, just like compressed springs.

SKIPPEN: As we started cutting and removing the first sections of steel, it was really challenging to figure out where the energy was stored in different members and how they would react.

BIANCHI: That stored energy of the bridge, I like to describe it like taking a metal Slinky that has all that energy that wants to come like this, but it's held with the steel.

When you cut that, all the energy is free and it'll break apart like that, and sometimes it's very dynamic and dramatic.

PINCHASIN: We have people, crane operators, operating very close to this massive Jenga, pick-up-sticks, loaded spring of a problem where members are being cut and then they spring back.

(metal scraping, water splashing) It's very dangerous.

SKIPPEN: It'll certainly make your hair stand up on the back of your neck seeing the steel drop as you're making cuts.

NARRATOR: To speed up the job and lift out the biggest sections of bridge possible, the team calls in the largest floating crane on the East Coast.

The Chesapeake 1000 is capable of lifting up to a thousand tons.

BIANCHI: You're not just dealing with weight, you're dealing with all the forces that are working against you.

There were days where we were battling specific sections due to mud suction.

You have this force that wants to keep it sucked down in the mud and you're trying to apply a force that's coming opposite of that, so you have to break that suction force.

And then as you're coming up out of the water, you have all this material just hanging off, dangerous road bed, rebar and stuff.

We had to get under there with small boats and torch cut and get that off of the structure.

So from start to finish, just a very inherently dangerous world that we live in.

NARRATOR: The salvage team hopes to recover the bodies of the four missing construction workers as they remove the layers of steel and concrete.

PINCHASIN: To get out onto the water, you go past the memorial, you see all the flags from their home countries and the people that are gathered there, the families that were gathered there.

When anyone was recovered, everything stopped, it was a very solemn and respectful moment.

We were able to then return those people to their families.

NARRATOR: The complexity of the salvage leaves Baltimore's main shipping channel completely blocked more than three weeks after the collision.

Hundreds of ships are unable to dock.

To get shipping moving again, engineers open three temporary channels for smaller vessels... ...to restore around 15% of commercial shipping to the port.

The next priority is to clear a 300-foot-wide section of the deeper main shipping channel that will allow single lane traffic for larger vessels, and restore around 70% of shipping to the port.

(seagulls squawking) It takes almost four weeks to haul this wreckage from the mud.

PINCHASIN: When they opened the limited access channel and you saw the scale of the ships that we were able to bring in we just knew that we were getting one step closer to bringing Baltimore back to full functionality.

NARRATOR: But this access channel creates another challenge for the team.

Larger vessels disturb the water so much, that the Dali could start to rock and roll.

There's a risk the vessel could break free, swing around and block the channel again.

The team urgently needs to stabilize the ship, so they pump water into the ballast tanks at the front to weigh down the bow and lock it into the riverbed.

To secure the stern, they drop four massive anchors, and they send in powerful tugboats to restrain the Dali should the ship break its chains.

As the team races to secure the vessel, they must also devise a plan to remove the 5,000-ton truss resting precariously across the bow.

The bridge section on the Dali was very unstable.

There was bolts falling off it, pieces of seal falling off it.

Taking care of the truss on the ship was gonna be challenging and dangerous.

NARRATOR: The cost of the port shutdown exceeds $15 million a day.

And this is not the first time in America that a ship colliding with a bridge has led to catastrophe.

(helicopter whirring) An incident more than 40 years earlier highlights the vulnerability of bridges, but also shows how to protect them from ship strikes.

In 1980, a freighter called "Summit Venture" brought down a section of the Sunshine Skyway Bridge in Florida.

ACCIDENT SURVIVOR: And as I came to the very top of the bridge here, I saw the rest of the bridge was out and applied the brakes immediately and I stopped within about two feet of going in.

NARRATOR: But 35 people lost their lives in this tragedy.

The new bridge was built with engineered islands to protect the main piers from ship collisions.

Vast underwater pillars called "dolphins" guard the piers that hold up the roadway.

AGHAYERE: Dolphins, they are really anchored all the way down into the seabed and filled with concrete, so they are massive, they're not pushovers.

They dissipate the energy and bring the ship to a stop.

NARRATOR: After 1991, all new bridges had to be built with pier protection.

Bridges built before then were not required to be protected.

They were kind of just left alone.

So that's why a bridge like the Francis Scott Key Bridge was left unprotected.

NARRATOR: But retrofitting pier protection to older bridges is possible.

Operators of the Delaware Memorial Bridge, which opened in 1951, decided to spend $93 million to install eight new dolphins.

It would cost almost $2 billion to replace the Delaware Memorial Bridge in the event of a collapse-- more than 20 times the cost of the dolphins.

In the 1970s, the Key Bridge was built with four dolphins, but they were all positioned over 400 feet from its main piers.

AGHAYERE: The dolphins were so far away from the piers that they were basically useless in terms of protecting the piers.

I believe that if you had dolphins, closer up dolphins, protecting the Key Bridge, that direct hit from the Dali would not have happened.

The ship would be damaged, but the bridge would have been spared.

The lives would have been spared.

Maritime law is written in blood.

We tend to wait for an accident to take place.

It took Titanic's sinking for us to come up with a convention that mandated enough lifeboats onboard.

It's because we can't imagine something as bad as what happens with Dali ever happening.

NARRATOR: Since the Francis Scott Key Bridge opened in 1977, thousands of ships have passed safely underneath.

But this disaster had been foreseen.

Between 2006 and 2016, meeting notes reveal that Baltimore's Harbor Safety Committee discussed the need for the Key Bridge to be protected from ship strikes.

There were people really stressing that there needs to be pier protection.

You could have ships lose electrical power, and if they lose electrical power, they could drift and, and hit the piers.

But the issue of cost is discussed, and it seems crazy but nothing was done.

There was a real opportunity to prevent what happened.

NARRATOR: The Dali is not even the first ship to strike Baltimore's Key Bridge.

On August 29, 1980, a 400-foot-long cargo ship also lost power leaving port, and slammed into the same pier that the Dali would hit over four decades later.

REPORTER: Here's where it happened, a Japanese ship called the Blue Nagoya was heading south out of Baltimore Harbor when it rammed right into Key Bridge; as you can see, it tore off about 30 feet of planking and damaged part of the bridge's support structure.

NARRATOR: The Blue Nagoya was a large ship for 1980, but it was only about a third the size of the Dali.

Even so, the ship caused half-a-million dollars' worth of damage to the Key Bridge.

AGHAYERE: The fender was badly damaged, and had to be replaced, so that was a warning sign.

It was something that they knew had happened before.

So if it had happened before, the probability that it will happen again was always there.

NARRATOR: But the damage the Dali would cause on March 26, 2024, was completely unprecedented.

That morning, the ship was on the correct course to sail directly between the main piers of the bridge.

So what caused it to veer to the right and head straight for the bridge pier?

The ship's data recorder reveals that Dali lost power at 1:25 in the morning, and started to drift rudderless at nine knots.

At that time, the vessel was sailing closer to one side of the shipping channel, possibly causing pressure differences along the ship's hull that may have pushed the bow to the right.

At that precise moment, the ship passes the mouth of a river tributary, whose currents may have pushed the stern to the left and set the ship on a collision course with the bridge pier.

With no propulsion to correct its course, the 124,000-ton juggernaut was only seconds from disaster.

MERCOGLIANO: If they lost power 30 seconds earlier, 30 seconds later, you probably don't have the collapse of the Key Bridge.

30 seconds earlier, the ship may have side swiped the pylon, it may have gone aground inside the bridge.

30 seconds later, it may have coasted under the bridge and not hit it.

NARRATOR: It's now over six weeks since the Francis Scott Key Bridge collapsed.

As teams prepare to move the stranded Dali, they continue to hunt for the sixth and final missing construction worker.

PINCHASIN: The recovery never stopped until every single person was found.

NARRATOR: Finally, with most of the mangled mass of steel out of the way, they must remove the truss pinning down the Dali so they can tow the vessel away and clear the shipping channel.

This will be a huge challenge.

There's so much pressure, compressive pressure, on the beam from how it's laying; that's very dangerous if you have someone in the way and all of a sudden it's gonna give way.

So the best thing to do is to remove the human from the equation.

Using controlled demolition was the fastest and safest way to conduct that many cuts, all at the same time.

NARRATOR: Workers makes cuts at strategic points, but do not fully sever the beams.

Then they insert specially shaped charges, explosives that create high velocity jets, to slice through the steel and complete the cuts.

The charges must now explode, so the 5,000 ton truss section falls safely away from the ship.

It's now seven weeks since the disaster.

The Dali's crew continues to assist with the investigation, and visa restrictions still prevent them from leaving the ship.

With safety measures in place, they are moved to the stern, away from the blast.

This is a critical operation engineers have been meticulously preparing for.

But have they thought of everything?

Are all their calculations correct?

BIANCHI: Blast day was set.

The time was set.

We had a blast radius, so we had to stay out, outside of and you know, a safety zone set up.

O'CONNELL: We were telling people it wasn't gonna be as you see in the movies, a big explosion.

But at the end of the day, it turned out to be a pretty big explosion.

(explosion booms) BIANCHI: Feeling the boom and then seeing it fall in the water, it was pretty magnificent, it was, it was pretty cool.

I mean, I, I've never seen anything like it before.

(explosion booms) But then I was thinking, "All right, what do we have to do to go get this, like, out of the water?"

NARRATOR: Finally, after eight weeks of cutting and lifting wreckage from the river, they free the ship from the metal.

They pump water out of the ship's ballast tanks to help raise the Dali off the riverbed.

And that's when she came free, and floated up and we were able to pull her off.

NARRATOR: A fleet of 5,000 horsepower tugs helps haul the ship away from the devastated pier.

Finally, 11 weeks after the accident, the salvage team removes the last of the wreckage, fully restoring shipping to the Port of Baltimore.

BIANCHI: Seeing the first deep draft ship come through that channel, I felt like there should have been fireworks behind it; you know, it was an accomplishment for everyone.

NARRATOR: This highly coordinated, complex salvage operation clears the way for the Port of Baltimore to reopen.

It costs over $100 million to clear the wreckage, and it could cost almost $2 billion more to replace the Key Bridge.

A number of lawsuits against the Dali's owners allege that poor maintenance and cost-cutting cause the ship to lose power and hit the bridge.

The ship's owners reject these allegations, but if upheld, maritime insurers estimate that the total claims could approach $4 billion.

Ever since the crash, the National Transportation Safety Board's lead investigator, Marcel Muise, has been working to find out what caused the Dali to lose power.

His team has now released preliminary reports that suggest how a series of unfortunate events may have led to this tragedy.

MUISE: Why the ship lost power is absolutely the focus of the investigation, we want to prevent this from happening again.

Ships are complicated.

They need electrical power to run a variety of systems, including propulsion and steering and navigation.

NARRATOR: The investigation reveals that on the night of the crash, the Dali was running two of its four power generators.

Together, they produce over eight megawatts of electricity to power nearly everything on the ship, including the fuel and water pumps for the main engine and the hydraulics of the steering mechanism.

MERCOGLIANO: Ships generate a great deal of electricity to run, and much like your own house where you have circuit breakers to ensure that if there is a disruption or a problem, they'll trip, ships have them too.

NARRATOR: The investigation discovers a loose cable, which lawsuits claim was shaken loose by engine vibrations, causing a power surge that tripped two breakers, disrupting the electrical power supply.

The system should have automatically switched to a secondary circuit.

But lawsuits allege the control was set to manual, so did not automatically switch, causing a blackout across large parts of the ship.

The pumps required to run the main engine and the steering all shut down, which killed the propulsion and locked the rudder in place.

MUISE: When they were about three ship lengths away from the bridge, the ship lost power.

MERCOGLIANO: Having sailed for many years on a ship, I can tell you that the worst sound you ever hear on a ship is silence.

When everything goes quiet and dark, it is the worst feeling because at that moment you realize you don't have control of the ship anymore.

It is a nightmare, a nightmare scenario to be up on a ship when it loses power in close quarters.

You have a very short period of time in which to try to restore power.

From the time period that Dali lost power till it hit the bridge was four minutes.

NARRATOR: A small emergency generator kicked in, and the crew closed the tripped breakers.

Lighting came back on, but the engine was still offline.

MERCOGLIANO: When the ship goes dark, the main propulsion engine of the ship also cuts off, it's not getting the fuel, the lube oil, the water.

You basically have stalled that engine.

NARRATOR: But before the crew had a chance to restart the engine, there was a second blackout.

Investigators discover that the generators caused this outage.

One lawsuit claims that after the first blackout, the pump supplying the generators with fuel shut down, and even when the power came back on, this pump did not restart automatically.

The generators would then have lacked fuel pressure, which could have caused them to run erratically, creating power fluctuations that trip the breakers and cause the ship to go dark again.

They never were able to get propulsion going again.

After the second blackout, the ship was about 1.6 ship lengths away from the bridge.

NARRATOR: The timing of the blackout was exceedingly unlucky, but the blackout itself was far from unusual.

Ship blackouts happen surprisingly frequently.

On average, over five large vessels lose power every week in the U.S. alone.

Fortunately, few of these result in collisions.

MERCOGLIANO: Most of the time these blackouts occur at sea, when the ship is sailing out in the ocean.

It takes a bit of time to find the underlying cause that is creating the fault.

And as Dali was approaching the Francis Scott Key Bridge, the one thing that crew did not have was time on their hands.

NARRATOR: The Dali's log reveals that in a last ditch attempt, the crew tried to power up the bow thrusters and drop an anchor.

But the thrusters were offline.

And the break to lock off the anchor could not be applied.

MERCOGLIANO: By the time the anchor was dropped, they were coming up on top of the bridge, the crew member on the bow had to evacuate before Dali hit the bridge.

The ship slowly drifted to starboard, and they contacted pier 17 of the bridge.

NARRATOR: The NTSB's preliminary report reveals the night of the accident was not the first time that the Dali had experienced blackouts.

The day before, while still in port, the ship lost power twice.

This resulted in power being rerouted to a second circuit which had not been used for months.

This is the circuit with the loose cable that's alleged to have possibly caused the fateful blackout the following morning.

(electricity buzzing) This loss of electrical power triggered a cascade of disasters that affected the lives of countless people.

None more so than the construction workers who were fixing potholes on the Key Bridge that night.

MOORE: I know the journey of healing for Baltimore is gonna be a long road.

Uh, I, I know that for those families who lost individuals, this is a burden they will feel for the rest of their lives.

NARRATOR: Six lives were lost in the collapse of the Francis Scott Key Bridge.

But the accident could have been far worse.

MERCOGLIANO: You have to remember, this didn't happen during rush hour in Baltimore when that bridge has hundreds, if not thousands of people going over it.

NARRATOR: When the Dali first lost power, one of the ship's pilots put out a distress call.

Maryland Police were controlling traffic around a lane closed off for the roadworkers, so quickly began to shut down bridge traffic.

Cars would have kept coming, had they not stopped traffic and by the time the people driving the vehicles would have realized the bridge was gone it just would have been too late.

I can't tell you how many lives they saved, because the answer's countless.

NARRATOR: The last moving vehicle cleared the bridge just 40 seconds before the Dali brought it crashing down.

POLICE OFFICER: We got a call, I mean, like, probably a minute if that, before it hit it.

NARRATOR: Unfortunately, the police were unable to reach the construction workers to warn them of the danger.

NARRATOR: The Francis Scott Key Bridge only stood for 47 years, but saw profound changes to shipping.

MERCOGLIANO: In 1977, very few people foresaw a ship the size of Dali.

What we've seen is kind of the exponential increase in the amount of goods we move by sea.

Bridges and safety measures have to keep pace with the way we're changing ocean shipping.

NARRATOR: An estimated 2,500 U.S. bridges remain at risk of ship strikes, including the 72-year-old Chesapeake Bay Bridge, just 20 miles south of Baltimore.

AGHAYERE: We can't let our old bridges be sitting ducks.

We can't just leave them unprotected.

Structural pier protection, having tug boats lead the ships, but doing something, not just sitting and waiting and hoping that nothing happens.

NARRATOR: The National Transportation Safety Board has been calling on the Coast Guard and the Federal Highway Administration to review the adequacy of bridge pier protection since 1988.

Now, an urgent assessment of ship collision risk is underway across the nation.

It's possible that recommendations will include vast investment to safeguard the country's bridges.

And in the end, many hope the lessons learned from the tragic loss of the Key Bridge will not be forgotten.