Part of the Francis Scott Key Bridge in Baltimore collapsed early Tuesday after a column was hit by a large container ship, sending cars and possibly people into the Patapsco River, authorities said.

The Baltimore City Fire Department told CBS News that at about 1:30 a.m., 911 calls started coming in saying a vessel had hit a bridge column. It was unclear how many vehicles were on the span but "there was surely a large tractor-trailer" on it. 

"There are at least seven people" in the river and "the dive and rescue team has arrived on scene to locate the individuals."

The department described the collapse as "a mass casualty event."

A spokesperson told The Associated Press it was a "dire emergency." Kevin Cartwright, the department's director of communications, said some cargo was apparently dangling from the bridge.

The department said on its scanner that there was lots of diesel fuel in the water around the ship and that there were construction workers on the bridge pouring concrete at the time.

A Baltimore police spokesperson told CBS News there was a "partial bridge collapse, with workers possibly in the water."

The MidAtlantic Coast Guard told CBS News the 948-foot Singapore-flagged cargo ship Dali had struck the bridge. It was apparently en route from Baltimore to Colombo, Sri Lanka.

The AP reported that the ship caught fire and sank.

Reuters reports that the ships owner, Synergy Marine Group, said the cause of the collision wasn't known but all crew members were accounted for and there were no reports of any injuries to any of them. 

Baltimore Mayor Brandon M. Scott said on social media that. "I'm aware of and en route to the incident at the Key Bridge. ... Emergency personnel are on scene, and efforts are underway."

Baltimore County Executive Johnny Olszewski Jr. said on social media that, "Rescue efforts are underway. Please pray for those impacted." 

The Maryland Transportation Authority said all lanes were closed in both directions for an incident on I-695 Key Bridge and later posted that traffic was being detoured to I-95 and I-895.

The Patapsco River, which the bridge crossed, is a key waterway that serves with the Port of Baltimore as a hub for East Coast shipping.

The bridge is named after the writer of "The Star-Spangled Banner." It was opened in 1977.