Baltimore is facing a possible “mass casualty” incident after a ship collided with the Francis Scott Key Bridge early Tuesday, causing the bridge to collapse and sending as many as 20 people into the frigid water, a fire official told CNN.

Divers and search and rescue teams are searching for people in the Patapsco River after several vehicles are believed to have fallen into the water, including one as large as a tractor-trailer, said Kevin Cartwright, director of communications of the Baltimore City Fire Department.

Local agencies received 911 calls around 1:30 a.m. reporting that a ship, traveling outbound from Baltimore had struck a column on the bridge, Cartwright said.

There could have been a vehicle “as large as a tractor trailer” on the bridge at the time of collapse, he said.

Teams were working “methodically and safely” to ensure “everyone operating here on the scene is safe and that we’re able to make progress without causing adverse harm to anyone else,” Cartwright said.

All traffic is being detoured, said the Maryland Transportation Authority.

Baltimore Mayor Brandon Scott said in a social media post around 3 a.m. that he was en route to the incident at the bridge. “Emergency personnel are on scene, and efforts are underway,” he added.

Video of the collision shows a towering boat headed directly toward one of the bridge’s support columns before colliding with it, sending a massive stretch of the bridge crashing into the water below in mere seconds. The impact immediately triggered the collapse of adjacent portions of the bridge.

The crash sent large plumes of smoke and fire into the air and part of the bridge appeared to collapse over the front of the boat, video footage showed. Dark smoke continued to rise into the air for several minutes.

The 1.6-mile, 4-lane bridge extends over the Patapsco River and serves as the outermost crossing of the Baltimore harbor and an essential link of Interstate-695, or the Baltimore Beltway.

Its namesake, Francis Scott Key, is believed to have sat near the site of the bridge as he witnessed the bombardment of Fort McHenry in 1814, inspiring him to write the words of the “Star Spangled Banner.”