As federal investigators try to determine why an Amtrak train that crashed in Philadelphia sped up in the minute before it derailed, Amtrak's top official said the national passenger railroad takes full responsibility for the deadly wreck.

Joseph Boardman, Amtrak CEO, said in a letter on Amtrak's official blog on Thursday that it is cooperating fully in an investigation into the accident that killed eight people and injured more than 200 on Tuesday.

"With truly heavy hearts, we mourn those who died. Their loss leaves holes in the lives of their families and communities," Boardman wrote. "Amtrak takes full responsibility and deeply apologizes for our role in this tragic event."

Meanwhile, the US National Transportation Safety Board said on Thursday that the Washington-to-New York train had inexplicably sped up from about 110 km per hour to more than 160 km/h in the last 65 seconds before the crash, according to video from the locomotive's front-facing camera.

Board member Robert Sumwalt said it's unclear whether the speed was increased manually by engineer Brandon Bostian. The speed in the turn was supposed to be 80 km/h.

So far, investigators have found no problems with the track, the signals or the locomotive, and the train was on schedule, Sumwalt said.

AP - Reuters