WILLIAM BRANGHAM: Welcome to the "NewsHour."

Search-and-rescue efforts are under way in Baltimore after a major commuter bridge collapsed there overnight.

Officials say six people remain unaccounted for.

In the middle of the night, a container ship slowly trying to exit Baltimore's outer harbor rams into a support beam of the Francis Scott Key Bridge, and, all at once, the towering 1.6-mile-long structure plummets into the river below.

BRANDON SCOTT (D), Mayor of Baltimore, Maryland: Never would you think that you would see, physically see, the Key Bridge tumble down like that.

It looked like something out of an action movie.

WILLIAM BRANGHAM: By sunrise, the scope of the disaster came into full-view, the on-ramp severed, the cargo ship buried beneath the bridge's mangled truss.

Investigators are still seeking answers to what happened.

Maryland Governor Wes Moore: GOV.

WES MOORE (D-MD): I know this has been a long night.

We started coordinating immediately after the Key Bridge collapsed.

The preliminary investigation points to an accident.

We haven't seen any credible evidence of a terrorist attack.

WILLIAM BRANGHAM: A few minutes before impact, the ship's lights went out.

Operators put out a mayday that the vessel had lost power, giving police time to divert traffic from the bridge before the collision.

GOV.

WES MOORE: Many of the vehicles were stopped before they got onto the bridge, which saved lives in a very, very heroic way.

WILLIAM BRANGHAM: Still, sonars detected several cars in the water.

And authorities say a crew had been repairing potholes on the bridge when it collapsed.

Maryland's secretary of transportation said a massive search-and-rescue is under way.

PAUL WIEDEFELD, Maryland Secretary of Transportation: We're basically searching for everyone that was potentially on the bridge.

As you can imagine, it's the middle of the night.

What type of traffic was there?

How many workers were there?

WILLIAM BRANGHAM: President Biden said today that he intends to have the federal government pay to rebuild the bridge.

JOE BIDEN, President of the United States: This is going to take some time.

And the people of Baltimore can count on us, though, to stick with them at every step of the way.

WILLIAM BRANGHAM: The Key Bridge was one of three interstate crossings of the Patapsco River, which is a tributary of the Chesapeake.

The remaining two are both lower-clearance tunnels.

The wreck has already snarled commuter traffic, as well as shipping traffic in and out of Baltimore.

But experts say other harbors along the East Coast can handle the pressure.

EMILY STAUSBOLL, Analyst, Xeneta: Baltimore is not one of the bigger container ports in the region.

WILLIAM BRANGHAM: Emily Stausboll is an analyst at Xeneta, an international shipping analytics platform.

EMILY STAUSBOLL: The ports of New York, Jersey and Virginia are much bigger and also have -- at the moment have capacity to be able to manage diverted volume from Baltimore.

If you combine those three ports in 2023, about 10 percent of containers were handled in Baltimore.

WILLIAM BRANGHAM: For now, the harbor is dotted with remnants of a bridge, used by 30,000 drivers daily, with no timeline on when it will be operational again.