The bridge is an important cog in the region’s transportation infrastructure, and its collapse is expected to snarl commuter traffic and disrupt a vital shipping port.

It was the middle of the night when a dispatcher’s 12-second warning crackled over the radio: A massive cargo ship had lost its steering capabilities and was heading toward the Francis Scott Key Bridge.

Within about 90 seconds, police officers responded that they had managed to stop vehicle traffic over the Baltimore bridge in both directions. One said he was about to drive onto the bridge to alert a construction crew.

But it was too late.