PHILADELPHIA- Rescue workers today searched through twisted metal and debris after an Amtrak train derailed in Philadelphia, while investigators sought the cause of the accident that killed at least six people and injured scores of others.

Authorities said they did not know why the New York City-bound train carrying 243 people derailed at about 9:30 p.m. Tuesday (0130 GMT Wednesday), sending all seven cars and the engine off the track. One car was tossed upside down and three on their sides, and passengers and luggage were sent flying, survivors said.

Philadelphia-area hospitals and health systems reported treating about 135 people. Philadelphia Mayor Michael Nutter said authorities had not yet accounted for everyone aboard the train.

One passenger, Daniel Wetrin, told CNN by phone that he was riding in the last car when it derailed and he was thrown into the aisle.

"Chairs were flying around, people were flying, bags, (it was) pretty chaotic," he said. "There were two people above our heads in the luggage rack."

Moments later, passengers in his car managed to open a rear door and escape, he said, adding he was unaware of any major injuries.

In a video posted on social media, passengers could be heard crying while others told victims to crawl forward.

"It's an absolute disastrous mess," Nutter said. "I've never seen anything like this in my life."

The crash of Amtrak train No. 188, en route from Washington, D.C. with a crew of five, was the latest in a series of rail accidents on heavily traveled passenger train routes over the past year.

The train derailed in the city's Port Richmond neighborhood along the Delaware River, near the site of a 1943 rail accident that killed 79 people.

Amtrak, a publicly funded national passenger rail line, provided no further details about the accident. It said Amtrak service along its busy Northeast corridor between New York and Philadelphia had been suspended.