UPDATE | Investigators recover black boxes after Amtrak train wreck kills 6

PHILADELPHIA (AP) — Rescue crews searched the mangled wreckage for victims today as investigators tried to determine why an Amtrak train hurtled off the tracks in a crash that killed at least six people, injured more than 200 and plunged screaming passengers into darkness and chaos.

Investigators recovered the train's data recorders and said they expected them to yield crucial information, including how fast it was going as it rounded a sharp curve and derailed in an old industrial neighborhood not far from the Delaware River shortly after 9 p.m. Tuesday.

"It's a devastating scene. There are many first responders out there. They are working. They are examining the equipment, seeing if there are any more people in the rail cars," Robert Sumwalt of the National Transportation Safety Board said.

Mayor Michael Nutter said some people remained unaccounted for, raising fears the death toll could rise, 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.

The dead included an employee of The Associated Press and a midshipman at the U.S. Naval Academy.

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

The train was en route from Washington to New York with 238 passengers and five crew members listed aboard when it lurched to the side and flew off the tracks at a notorious curve not far from the scene of one of the nation's deadliest train wrecks more than 70 years ago.