Rescue workers on Wednesday sifted through twisted metal and debris from the wreck of an Amtrak train that derailed in Philadelphia, killing six people and injuring scores of others, as investigators began reviewing data to determine the cause of an accident.

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

Philadelphia-area hospitals and health systems reported treating more than 200 people, Philadelphia Emergency Management Director Samantha Phillips said at a news conference.

Mayor Michael Nutter said six people were confirmed dead, but authorities had not yet accounted for everyone aboard the train.

Officials did not provide a figure on the number of injured, but said the train's conductor was among them. The mayor said at the news conference that the conductor was giving a statement to police.

Officials of the National Transportation Safety Board had recovered recorders, or black boxes, from the train and were analyzing data on its speed and other operational factors, Robert Sumwalt, a NTSB member, said at the news conference. He said officials were also reviewing training records of the crew.

The agency, coordinating with the Federal Railway Administration, was also focusing on track conditions, equipment and human performance, among other factors, he said.

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.