PHILADELPHIA — The Amtrak train that derailed Tuesday, killing eight people and injuring more than 200, may have been struck by an object before it careened off the tracks, an assistant conductor on the train told investigators from the National Transportation Safety Board.

At a news conference on Friday, Robert L. Sumwalt, the safety board official who is leading the investigation, said an assistant conductor had reported that she believed she heard a radio transmission in which an engineer on a regional line said his train had been struck by a projectile and the engineer on the Amtrak train replied that his had been struck, too.

Mr. Sumwalt said that investigators had found a fist-size circular area of impact on the left side of the Amtrak train’s windshield and that they had asked the Federal Bureau of Investigation to analyze it. He said that the F.B.I. had been called in because it has the forensics expertise needed for the investigation, but that it had not yet begun its analysis.

He said that investigators had also interviewed the engineer and found him “extremely cooperative,” and that the engineer had said he was not fatigued or ill at the time of the accident. But he could not remember anything about the derailment.