The Amtrak train that crashed in Philadelphia, killing at least seven people, was hurtling at 170 km/h before it ran off the rails along a sharp curve where the speed limit is just 80 km/h, federal investigators said Wednesday.

The engineer at the controls applied the emergency brakes moments before the deadly crash but managed to slow the train to only 164 km/h when the locomotive's black box stopped recording data, said Robert Sumwalt of the National Transportation Safety Board. The speed limit just ahead of the bend is 130 km/h, he said. An earlier report cited the Federal Railroad Administration as saying the speed limit in that straight before the curve is 110 km/h.

The engineer, whose name was not released, refused to give a statement to law enforcement Wednesday and left a police precinct with a lawyer. Sumwalt said federal accident investigators hope to interview him but will give him a day or two to recover from the "traumatic event."

"Our mission is to find out not only what happened but why it happened, so that we can prevent it from happening again," Sumwalt said.

Sumwalt: Authorized speed in the curve was 50 mph. Speed when “engineer induced braking” was applied was 106 mph. <a href="https://twitter.com/hashtag/Amtrak?src=hash">#Amtrak</a> —@NTSB

More than 200 people aboard the Washington-to-New York train were injured in the wreck, which took place in a decayed industrial neighbourhood not far from the Delaware River shortly after 9 p.m. Tuesday. It was the nation's deadliest train accident in nearly seven years.

"We are heartbroken by what has happened here," Mayor Michael Nutter said.

Amtrak suspended all service until further notice along the Philadelphia-to-New York stretch of the nation's busiest rail corridor — forcing thousands of travellers to find some other way to reach their destination — as investigators examined the wreckage and the tracks and gathered up other evidence.

The dead included an AP employee and a midshipman at the U.S. Naval Academy. Many of the injured suffered from broken bones or burns.

Among the dead were award-winning AP video software architect Jim Gaines, a father of two; Justin Zemser, a Naval Academy midshipman from New York City; Abid Gilani, a senior vice president in Wells Fargo's commercial real estate division in New York; and Rachel Jacobs, who was commuting home to New York from her new job as CEO of the Philadelphia educational software startup ApprenNet.

At least 10 remained hospitalized in critical condition.

See cellphone clips from inside the train after the deadly crash in Philadelphia 2:15

Nutter said some people remained unaccounted for, though he cautioned that some passengers listed on the Amtrak manifest might not have boarded the train, while others might not have checked in with authorities.

"We will not cease our efforts until we go through every vehicle," the mayor said in the afternoon. He said rescuers expanded the search area and used dogs to look for victims in case someone was thrown from the wreckage.

The NTSB finding about the train's speed corroborated an Associated Press analysis done earlier in the day of surveillance video from a spot along the tracks. The AP concluded from the footage that the train was speeding at approximately 172 km/h moments before it entered the curve.

Despite pressure from Congress and safety regulators, Amtrak had not installed along that section of track Positive Train Control, a technology that uses GPS, wireless radio and computers to prevent trains from going over the speed limit, the railroad agency said.

.<a href="https://twitter.com/NTSB">@NTSB</a> says train control system that limits speed not installed in this area. if it was, NTSB says would have prevented crash <a href="https://twitter.com/hashtag/amtrakcrash?src=hash">#amtrakcrash</a> —@cbcsteve

Most of Amtrak's Northeast Corridor is equipped with Positive Train Control.

"Had such a system been installed in this section of track, this accident would not have occurred," Sumwalt said.

The notoriously tight curve is not far from the site of one of the deadliest train wrecks in U.S. history: the 1943 derailment of the Congressional Limited, bound from Washington to New York. Seventy-nine people were killed.

Amtrak inspected the stretch of track on Tuesday, just hours before the accident, and found no defects, according to the Federal Railroad Administration. In addition to the data recorder, the train had a video camera in its front end that could yield clues to what happened, said NTSB member Robert Sumwalt.

Passengers scrambled through the windows of torn and toppled cars to escape. One of the seven cars was severely mangled.

Amtrak carries 11.6 million passengers a year along its busy Northeast Corridor, which runs between Washington and Boston.

Video I took moments after crash. People trying to open the door and get off the train. <a href="http://t.co/fJ2bxXfmhG">pic.twitter.com/fJ2bxXfmhG</a> —@Janelle_News

Passenger Jillian Jorgensen, 27, was seated in the quiet car — the second passenger car — and said the train was going "fast enough for me to be worried" when it began to lurch to the right.

The train derailed, the lights went out and Jorgensen was thrown from her seat. She said she "flew across the train" and landed under some seats that had apparently broken loose from the floor.

Jorgensen, a reporter for the New York Observer who lives in Jersey City, N.J., said she wriggled free as fellow passengers screamed. She saw a man lying still, his face covered in blood, and a woman with a broken leg.

She climbed out an emergency exit window, and a firefighter helped her down a ladder to safety.

"It was terrifying and awful, and as it was happening it just did not feel like the kind of thing you could walk away from, so I feel very lucky," Jorgensen said in an email to The Associated Press. "The scene in the car I was in was total disarray, and people were clearly in a great deal of pain."