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Seth Wenig, Associated Press
Authorities wearing Federal Railroad Administration vests look over the wreckage of a Metro-North Railroad train and a SUV in Valhalla, N.Y., Wednesday, Feb. 4, 2015. Five train passengers and the SUV’s driver were killed in Tuesday evening’s crash in Valhalla, about 20 miles north of New York City. Authorities said the impact was so forceful the electrified third rail came up and pierced the train.

VALHALLA, N.Y. — Federal investigators arrived Wednesday at the site of a fiery commuter train crash that killed six people, looking for clues to why the SUV that triggered the wreck was stopped on the tracks.

As a National Transportation Safety Board team traveled from Washington to the crash site, local officials worked to identify those killed in the deadliest accident on one of the nation's busiest commuter rail lines — one that has come under harsh scrutiny over safety after a series of accidents in recent years. Fifteen people remained hospitalized, seven with very serious injuries, as officials said they were, for now, mystified by the ghastly crash on the Metro-North Railroad.

"It's really inexplicable, based on the facts we have now," Gov. Andrew Cuomo said on WCBS-AM radio.

"Everybody wants to know exactly what happened, so that if something can be corrected, we correct it," he said, but he added that it was "too early to say if there's anything to learn here."

Five train passengers — authorities initially said six — and the SUV's driver were killed in Tuesday evening's crash, in Valhalla, about 20 miles north of New York City. It happened in an area where the tracks are straight, and car traffic can be tricky, as drivers exiting or entering a parkway turn and cross the tracks near wooded area and a cemetery.

The driver had gotten out of her Mercedes SUV momentarily after the crossing's safety gates came down around her and hit her car, according to the driver behind her, Rick Hope. But then she got back in and moved forward into the tracks, he said.

"I said to myself, 'The clock is ticking here, the gate is down, the bells are ringing — what are you going to do here?'" he told WNYW-TV. " ... She looked a little confused, gets back in the car and pulls forward.

"It looks like she stopped where she stopped because she didn't want to go on the tracks" and perhaps was unaware that she was in front of the crossing gate already, he said.

Westchester County Executive Rob Astorino said it appeared that the woman got out of her car to lift the crossing gate off it.

Authorities said the impact was so forceful the electrified third rail came up and pierced the train, and Cuomo said the SUV's gas tank apparently exploded, starting a fire that consumed the SUV and the train's first car, which was left blackened, warped and mangled, its roof twisted sideways. The car, pushed about 400 feet, looked as though it was stuck on the front of the train.

Sitting in the first car, Christopher Gross was watching a movie on his laptop — "and all of a sudden, impact," he told ABC's "Good Morning America" on Wednesday. He was hurled onto the floor between two rows of seats, hearing screams and seeing flames about a foot from his head and a fellow passenger whose leg had been amputated below the knee.

"It's life or death at this point," he recalled feeling. And then, he said, a man whose own hands were burned elbowed open the emergency exit latch, allowing some of the train's roughly 700 passengers to escape.

Elsewhere, passengers found themselves trapped for a time in stifling cars as news of the fire spread, or climbing out of cars via ladders. The train's engineer tried to rescue people until the smoke and flames got so severe that he had to escape, Astorino said.

"I am amazed anyone got off that train alive," said Astorino, who used to commute on the same line. "It must have been pure panic."

It was unclear how fast the train was going, but the maximum would be 60 mph, a railroad official said.

Ryan Cottrell, assistant director at a nearby rock climbing gym, said he had been looking out a window because of an earlier, unrelated car accident and saw the train hit the car, pushing it along.

"The flames erupted pretty quickly," he said.

All railroad grade crossings have gate arms that are designed to lift automatically if they strike something like a car on the way down, railroad safety consultant Grady Cothen said. The arms are made of wood and are designed to be easily broken if a car trapped between them moves forward or backward, he said.

As of Wednesday morning, transit officials hadn't found any problems with the tracks or signal, Astorino said.

Metro-North is the nation's second-busiest commuter railroad, after the Long Island Rail Road. It was formed in 1983 and serves about 280,000 riders a day in New York and Connecticut. Service on its Harlem Line was suspended between Pleasantville and North White Plains after the crash.

Metro-North has been criticized severely for accidents over the last couple of years. Late last year, the NTSB issued rulings on five accidents that occurred in New York and Connecticut in 2013 and 2014, repeatedly finding fault with the railroad while also noting that conditions have improved.

Among the accidents was a Dec. 1, 2013, derailment that killed four people, the railroad's first passenger fatalities, in the Bronx. The NTSB said the engineer had fallen asleep at the controls because he had a severe, undiagnosed case of sleep apnea.

Last March, the Federal Railroad Administration issued a stinging report on Metro-North, saying it let safety concerns slip while pushing to keep trains on time. Railroad executives pledged to make safety their top priority.

Associated Press writers Jim Fitzgerald, Joan Lowy in Washington, and Jennifer Peltz and Ula Ilnytzky in New York contributed to this report.