The FBI is being called in to determine if a projectile struck a commuter train before it crashed in the US city of Philadelphia this week, killing eight people.
An assistant conductor aboard the locomotive reported that she heard the train's engineer say it was struck by something shortly before the crash, National Transportation Safety Board member Robert Sumwalt said during a press conference Friday.
The woman also said that she heard a report over her radio that a separate local train had been "hit by a rock or shot at," causing that engineer to make an emergency stop.
The NTSB said it had asked the FBI to provide "technical assistance" to examine the front of the derailed locomotive, where the train may have been hit.
Sumwalt described the possible impact area as a "circular pattern" radiating outwards on the left-hand side of the windshield.
The agency said on Twitter that it has also obtained video from the local train which it will examine.
Amtrak Train 188 was traveling from Washington to New York Tuesday when it crashed as it entered a curve, killing eight people and injuring more than 200.
The engineer, or train driver, has said he has no recollection of the last minutes before the derailment, as the seven-car train accelerated into a turn at twice the speed limit.
Sumwalt said investigators have not confirmed the reports of a projectile and had not drawn any conclusions about how the crash occurred. It is unclear how a projectile impact may have factored into the derailment.
"Reports of #Amtrak locomotive being struck will be subject to inspection and verification," the NTSB tweeted.
Sumwalt is scheduled to speak on several major US television networks Sunday.
Investigators say the 32-year-old engineer, identified by US media as Brandon Bostian, has been very cooperative and that he described what seemed to be a routine train operation leading up to the crash.