(NECN/NBC News: Jay Gray) - Home video shows the chaos just minutes after the commuter train jumped the tracks in The Bronx, killing four and injuring more than 60.
"Several people were trapped under cars and in the cars," said FDNY Chief Edward Kilduff.
As first responders rushed in, firefighters and police worked to rescue survivors.
"Some parts had to be cut away from the train in order to lift them out," said Kilduff.
Medical teams set up a field triage unit to treat the victims. At least 11 remain in serious condition Sunday.
"You could feel it starting to lean, and I was like, 'hey, what's going on?" said passenger Dennis O'Neil.
O'Neil says the next thing he knew, the car was on its side.
"It hit the curve real hard, and it flopped over and we slid down the hill," he said.
Lisa Delgado's cousin was also on-board.
"She hit her side of the head. She saw the woman in front of her go out through the other window," said Delgado.
Local, state, and federal investigators, led by the National Transportation Safety Board, are on the scene and searching for clues to what led to the fatal accident.
"Our mission is to understand not just what happened, but why it happened, with the intent of preventing it from happening again," said NTSB member Earl Weener.
There are initial indications speed may have played a factor as the train rolled from the tracks and eventually stopped just inches from the Hudson and Harlem Rivers.
Early reports say the conductor, who was injured in the accident, has said there was a problem with the brakes. The investigation is expected to take a week to ten days.