BILOXI, Miss. — A freight train smashed into a charter bus in Biloxi, Mississippi, on Tuesday, dragging the bus 300 feet down the tracks and leaving at least three people dead, authorities said.

Images from the scene show the bus, upright and still intact, straddling the tracks, with the CSX train pushed up against its left side.

The bus was carrying 50 people from Austin, Texas, Biloxi Police Chief John Miller said at a news conference. He said authorities believe the bus was stopped on the tracks at the time of the crash, but they don’t yet know why.

At least three people were killed, according to Biloxi city spokesman Vincent Creel, who said emergency responders were still removing injured people from the bus more than 30 minutes after the crash, pulling many through windows.

Witnesses told WLOX-TV that passengers were senior citizens. Charter buses often carry patrons to one of Biloxi’s eight casinos, but Creel said he wasn’t sure where the bus was headed.

“I want you to know we’re doing everything we can to get everyone off this bus,” Miller said.

The CSX train, pulled by three locomotives, was headed eastbound when it hit the bus at a crossing in downtown Biloxi just before 2:15p.m., pushing the bus about 300 feet down the tracks, Creel said. The single track is the CSX mainline along the Gulf Coast, passing through densely populated areas of southern Mississippi.

A nearby hospital set up a triage unit at the site to treat the injured, and helicopters were carrying the injured to hospitals that handle trauma cases.

The bus was marked as belonging to Echo Transportation, which Texas corporate records show is a unit of a company called TBL Group, based in Grand Prairie, near Dallas. A phone message and an email left for Echo were not immediately returned.

CSX, based in Jacksonville, Florida, didn’t immediately respond to a request for comment.