A freight train smashed into a charter bus in Biloxi, Mississippi, on Tuesday, pushing the bus 300 feet down the tracks and leaving at least four people dead, authorities said. Rescuers spent more than an hour removing passengers, cutting through the bus's heavily damaged frame to extract the last two.

Three died at the scene and one more later died in treatment, officials said.

Thirty-five people were injured, officials said. Two victims had to be extricated after becoming trapped in the bus. All others were able to be removed with varying degrees of injuries, officials said.

Seven people were uninjured in the crash, officials said.

Biloxi Police Chief John Miller said passenger on the Echo Transportation bus had come from Austin, Texas, carrying passengers to one of Biloxi's eight casinos. Ameet Patel, senior vice president of regional operations for Penn National Gaming, owner of Hollywood Gulf Coast Casino in Bay St. Louis and Boomtown Biloxi Casino, said the bus was traveling from the Hollywood casino to the Boomtown casino at the time of the crash.

An eye witness to the crash told the Biloxi Sun Herald that the bus had been stuck at a railroad crossing in downtown Biloxi for five to 10 minutes when the train hit it. Some passengers had been getting off of the bus when the collision happened causing some to become trapped under the bus, the eye witness said.

The bus could be seen straddling the tracks, with a CSX Transportation locomotive pushed up against its left side. The bus was apparently stopped on the tracks when the 52-car train, pulled by three locomotives, slammed into it, said Miller.

"We're not sure why," Miller said. "We don't know if there were mechanical issues or what was taking place."

The train was headed from New Orleans to Mobile, Alabama, at the time of the crash, said CSX spokesman Gary Sease. He said the train crew was not injured. The single track is the CSX mainline along the Gulf Coast, passing through densely populated areas of southern Mississippi.

Federal Railroad Administration spokesman Marc Willis said the agency is sending three inspectors to investigate, while Mississippi is sending one. The National Transportation Safety Board said it is also investigating.

The Associated Press contributed to this report