A container ship rammed into a major bridge in Baltimore, causing it to snap and plunge into the river below. Several vehicles fell into the chilly waters, and rescuers were searching for survivors.

Two people were rescued from the waters under the Francis Scott Key Bridge, one in serious condition, according to Baltimore Fire Chief James Wallace. He said authorities “may be looking for upwards of seven people” but said that number could change. It was not clear if the two rescued were included in the seven.

It was not immediately clear what caused the cargo ship to crash into the bridge long before the busy morning commute in what one official called a “developing mass casualty event” in a major American city just outside of Washington.

The ship crashed into one of the bridge’s supports, causing the structure to snap and buckle at several points and tumble into the water in a matter of seconds — a shocking spectacle that was captured on video and posted on social media. The vessel caught fire, and thick, black smoke billowed out of it.

“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,” said Baltimore Mayor Brandon Scott, calling it “an unthinkable tragedy.”

Sonar has indicated that there are vehicles in the water, where the temperature was about 47 degrees Fahrenheit (8 degrees Celsius) in the early hours of Tuesday, according to a buoy that collects data for the National Oceanic and Atmospheric Administration.

Earlier, Kevin Cartwright, director of communications for the Baltimore Fire Department, told The Associated Press that several vehicles were on the bridge at the time of the collapse, including one the size of a tractor-trailer truck. The bridge came down in the middle of night when traffic would be lighter than during the day when thousands of cars traverse the span.

Cartwright called the collapse a “developing mass casualty event,” though he didn’t know at the time how many people were affected.

He added that some cargo appeared to be dangling from the bridge, which spans the Patapsco River at the entrance to a busy harbor. The river leads to the Port of Baltimore, a major hub for shipping on the East Coast. Opened in 1977, the bridge is named for the writer of “The Star-Spangled Banner.”

Maryland Gov. Wes Moore declared a state of emergency and said he was working to get federal resources deployed. The FBI was also on the scene.

Synergy Marine Group — which owns and manages the ship called the Dali — confirmed the vessel hit a pillar of the bridge at about 1:30 a.m. while two pilots were in control. It said all crew members, including the pilots, were accounted for and there are no reports of any injuries.

The Dali was headed from Baltimore to Colombo, Sri Lanka, and flying under a Singapore flag, according to data from Marine Traffic. The container ship is about 985 feet (300 meters) long and about 157 feet (48 meters) wide, according to the website.

In 2001, a freight train carrying hazardous materials derailed in a tunnel in downtown Baltimore and caught fire, spewing black smoke into surrounding neighborhoods and forcing officials to temporarily close all major roads into the city.