PHILADELPHIA -- In a surprising revelation, the National Transportation Safety Board said a projectile appears to have hit the windshield of an Amtrak passenger train before it derailed Tuesday night, killing eight passengers and severely injuring dozens.

According to one witness, it happened shortly after a second train reported being hit by a rock or bullet near the same location.

Now, the FBI has been brought in.

At a news conference Friday evening, NTSB member Robert Sumwalt explained that investigators interviewed two assistant conductors from Amtrak Train 188.

One said that shortly after leaving Philadelphia's 30th Street station, she heard an engineer from a local SEPTA commuter train report his train being hit.

"She recalled that the SEPTA engineer had reported to the train dispatcher that he had either been hit by a rock or shot at," Sumwalt said. "And that the SEPTA engineer said that he had a broken windshield, and that he placed his train into emergency stop."

The assistant conductor went on to tell investigators she believed the Amtrak train had also been hit. Sumwalt described a circular pattern of damage on the lower port of the windshield.

The NTSB said it wants the FBI to examine damage to the windshield of Amtrak Train 188, to determine if the train may have been struck by a projectile prior to Tuesday night's derailment in Philadelphia. CBS News

"This is her recollection, and certainly we are going to be conducting further investigation of this comment," Sumwalt said. "Our investigation has not independently confirmed this information, but we have seen damage to the left hand, lower portion of the Amtrak windshield that we have asked the FBI to come in and look at for us."

The NTSB later told CBS News it was looking into reports that a third train, Amtrak Acela 2173, was also struck by an object on Tuesday night.

Amtrak engineer Brandon Bostian, who was at the controls Tuesday night, was interviewed for the first time Friday by NTSB investigators.

He was described as cooperative, and did not report any issues with the train or his own health, including fatigue.

Sumwalt said Bostian continued to insist he has no recollection of the crash or what occurred before it.

For investigators, that means the mystery remains regarding what caused the train to accelerate to more than 100 mph just before it derailed.