(dramatic music) - [Narrator] March 26th, 2024, (dramatic music continues) 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.

The ship's data recorder reveals that the 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.

- 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 sideswiped the pylon.

It may have gone aground inside the bridge.

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

(dramatic music) - [Narrator] 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.

(steel creaking) 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.

(dramatic music) - 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.

- You can see separation happening at the bottom.

The collapse is progressing actually, and you can see it 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.

- A typical progressive collapse scenario.

(dramatic music)