Speed was being eyed as a possible cause of the deadly Amtrak crash in Philadelphia, law enforcement sources told NBC News early Wednesday.
The derailment occurred on the same curve where one of America’s worst train disasters occurred 71 years ago.
The scene of Tuesday's crash was near a section of track where a 70 mph straight section went into a 50 mph curve — although officials stressed it was too early to know whether the curve, or speed, were factors.
A National Transportation Safety Board team arrived at the site early Wednesday and the Federal Railroad Administration said it was dispatching at least eight investigators.
“We do not know what happened here,” Philadelphia Mayor Michael Nutter told reporters. "We do not know why this happened. We are not going to speculate about it.”
However, he added that there was “one known fact … there is a curve.”
Train 188 was traveling to New York from Washington, D.C., and carrying about 238 passengers and 5 crew members when it derailed at around 9:30 p.m. ET., killing at least five people.
Law enforcement officials familiar with the Philadelphia regional and long-distance service identified speed as a likely cause of the crash, adding that whether human error or equipment failure were also involved might take more time to establish.
The crash unfolded very close to the site of the September 1943 Frankford Junction train wreck, in which a train from Washington, D.C., to New York derailed, killing 79 people and injuring 117 others. That unrelated disaster was blamed on mechanical failure: an overheated journal box that caused an axle to seize and snap.
Paul Cheung, who was aboard the train which derailed Tuesday, recalled that it "started to decelerate, like someone had slammed the brake."
Gaby Rudy, an 18-year-old from Livingston, New Jersey, said she was nearly asleep when she suddenly felt the train "fall off the track."
Transportation Secretary Anthony Foxx called it "a tragic accident" in a statement.
“Department of Transportation officials are already onsite, and we will work with NTSB to conduct a thorough investigation into the cause of this devastating event,” it said. "While there is still much we don't know, we at the U.S. Department of Transportation are deeply saddened by reports of multiple fatalities and injuries. Our hearts and prayers go out to the victims' families.”
Human error and track defects accounted for two-thirds of all railroad accidents last year, according to Federal Railroad Administration figures.
However, the number of accidents fell by 13 percent between 2011 and 2014, and the number of fatalities dropped by 67 percent over the same period. The overall railroad accident trend is downward, the FRA says.
The Associated Press contributed to this report.