Investigators from the National Transportation Safety Board are at the site of an Amtrak crash in Philadelphia to try to determine the cause of Tuesday night's derailment, responsible for at least six deaths, just as officials said they had recovered the train's black box.
The preliminary focus of the probe is speed, officials confirmed to ABC News today.
The train's black box -- known as an event recorder -- has been recovered, Philadelphia Mayor Michael Nutter said at a news conference today, and is now being analyzed at Amtrak's operation center in Delaware.
Nutter said the investigation is ongoing and will take some time. He did not offer an exact timeline.
The accident happened at about 9:28 p.m. when six cars overturned and the engine separated from the rest of the train.
Amtrak said there were about 238 passengers and five crew members on board. Five people died at the accident scene, while a sixth person died at Temple University Hospital, Chief Medical Officer Dr. Herbert Cushing said. Officials fear more bodies are in the wreckage. Hundreds of people were treated at area hospitals and 10 people remain in critical condition.
Asked how many people were unaccounted for, Nutter said officials "did not have an accurate count on that."
The train operator is alive, officials confirmed to ABC News. Amtrak service between New York and Philadelphia is shut down today.
Deborah Hersman, former chair of the NTSB, says black boxes and possible video cameras may provide important information.
"The black boxes will tell them things like speed, throttle position, braking. ... But also there are often inward and outward facing video cameras that will tell them a lot of information about the track conditions and the environmental conditions," Hersman said. "The key to the team is going to be grabbing those recorders right away.”
Investigators will also look at all other trains that traveled the same section of track, Hersman said.
The front end of the Northeast Regional train, which was traveling from Washington, D.C., to New York City, reportedly shook as it went into a turn and the train's seven cars went off the rails. The FRA reports that the engine of the train and two cars were upright, three were lying on their sides, one was nearly on its roof and one was leaning dramatically.
First responders used hydraulic tools to help trapped passengers out of the mangled cars.
Former Pennsylvania Rep. Patrick Murphy was on the train and he tweeted that he was all right and was trying to help others. He told ABC's Philadelphia station, WPVI-TV, that he heard a "bang," and the train "wobbled" from one side to the other.
pic.twitter.com/GvooK8lq58
— Patrick J. Murphy (@PatrickMurphyPA) May 13, 2015
"Obviously, there was a lot of mayhem," he told WPVI. "People were pretty banged up. There was a lot of blood, a lot of bleeding. I pulled myself up. The guy who I kind of landed on was OK. The guy next to him was completely passed out, knocked unconscious."
Murphy said the unconscious man came to, got off the train and began to help other people. Murphy said to his knowledge, most people had gotten off the train as of 11:15 p.m.
Paul Cheung, an Associated Press employee who was on the train, told the AP it appeared the train went off the tracks. He said he was fortunate to be at the back of the train and the front "looks pretty bad."
"The front of the train is really mangled," he said. "It's a complete wreck. The whole thing is like a pile of metal."
Delaware Sen. Tom Carper said he got off the train in Wilmington, just before the crash.
Carper said in a statement: "I am grateful to be home safe and sound in Wilmington, and my heart goes out to all those on the train tonight. I hope all of those that are injured recover quickly, and I will keep them in my thoughts and prayers."
ABC News’ Pierre Thomas contributed to this story.
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