WASHINGTON (AP) — An Amtrak train that derailed Tuesday evening in Philadelphia was travelling about 107 miles per hour as it approached a curve where the speed limit less than half that, according to an analysis by The Associated Press.

Surveillance video viewed by the AP shows the train — which was roughly 662 feet long — passed the camera in just over five seconds. But because the video inexplicably plays back slightly slower than in real time, it took the train a little more than four seconds to move past a fixed point on the screen.

That means the train was travelling about 107 miles per hour just before it derailed and tipped over, tearing the cars apart and killing at least seven people. More than 140 people went to hospitals to be evaluated or treated from the crash, and several were critically injured.

The surveillance camera from which the AP reviewed video was located at an industrial building a few hundred feet before a bend in the tracks, where the speed limit was only 50 miles per hour.

Light from an apparent explosion or a brilliant electrical discharge is visible in the video just over three seconds after the train passes. That would indicate the train was entering the curve as it began to derail, with the train located several hundred feet east of the camera.

The AP reviewed both the surveillance footage and government mapping data to determine the train's speed and location. That data showed the relative location of the surveillance camera to the train's path, the bends in the railroad tracks and the eventual location of the crash site.

Investigators have since recovered the locomotive's data recorder and said they expected it to yield crucial information, including how fast the train was going when it jumped the tracks in an old industrial neighborhood not far from the Delaware River shortly after 9 p.m. Tuesday.

The Amtrak engineer at the train's control refused to talk to police Wednesday and declined to provide a statement to authorities.