MAY 15, 2015

PHILADELPHIA — An assistant conductor on the Amtrak train that derailed on Tuesday believes she heard the engineer tell another regional train operator in a radio transmission that the train had been struck by something just before the accident, according to the National Transportation Safety Board.

At a news conference on Friday, Robert L. Sumwalt, the safety board official who is leading the investigation, said the F.B.I. had been asked to examine what he described as a fist-size impact area found on the lower left side of the train’s windshield. Officials 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.

Mr. Sumwalt said investigators had interviewed three crew members, including the engineer, whom they said was “extremely cooperative” and displayed good knowledge of the proper procedures and speeds for the rail line. He said the engineer had not been fatigued and was not ill.

“He recalls ringing the train bell as he went through North Philadelphia Station, as required,” Mr. Sumwalt said. “He has no recollection of anything past that.”

The assistant conductor, however, who was working in the cafe car, heard the engineer talking to his counterpart on a SEPTA regional line, who said his train had been “hit by a rock or shot at,” according to Mr. Sumwalt. The assistant conductor said she thought she heard the Amtrak engineer, Brandon Bostian, reply that his train had also been struck.

“Right after she recalled hearing this conversation between her engineer and the SEPTA engineer, she said she felt a rumbling, and her train leaned over and her car went over on its side,” Mr. Sumwalt said.

SEPTA — the Southeastern Pennsylvania Transportation Authority — said earlier this week that an “unknown projectile” had struck the engineer’s window of a train from Trenton at about 9:10 p.m., shortly before the Amtrak derailment. No injuries were reported, but the train was held at a nearby station because of the damage.

The Philadelphia Inquirer reported that another Amtrak train was struck on a coach window about 30 minutes before the crash, before the regional train was hit.

The train that derailed, Amtrak’s Northeast Regional train from Washington to New York City, was traveling over 100 miles an hour, more than twice the speed limit at that point, when it left the tracks, killing eight people and injuring more than 200.