Investigators have asked the FBI to look at whether a projectile, like a rock or bullet, hit Amtrak Regional 188 moments before it sped up and derailed at a curve in Philadelphia Tuesday night, killing eight people and injuring more than 200.

The latest information comes after NTSB investigators interviewed three Amtrak employees — including 32-year-old engineer Brandon Bostian — on Friday.

Bostian remembers ringing the train's bell as it passed through North Philadelphia station, about 4 miles from the crash site, but his memory is blank after that point, NTSB's Robert Sumwalt said. The interview lasted an hour and a half and Bostian, accompanied by his lawyer, was described as "extremely cooperative."

An assistant conductor told the NTSB she heard a SEPTA operator on adjacent tracks radio that his window had been shattered by an unknown projectile. That SEPTA train was one of two trains that NBC10 previously reported were hit by projectiles on the same line around the same time that night.

More than 40 Iraqi soldiers were killed after ISIS militants dug a mile-long tunnel under a local army headquarters and planted around 300 explosives under the base, a top regional official told NBC News Friday.

The woman, who was working in the fourth car that is a cafe car, said she believed she heard Bostian tell the SEPTA engineer their train had also been hit by something.

Moments later, the train careened off a curve at Frankford Junction in Port Richmond as it traveled more than 100 mph in a 50 mph zone.

"We have not independently confirmed this knowledge. We rely on the FBI for their expertise in such areas," Sumwalt said.

Bostian, who suffered a concussion, head and leg injuries and doesn't remember the crash, did not tell investigators the train was hit by a projectile. A second assistant conductor, interviewed Friday, did not hear the conversation either, but may have been having trouble with his radio.

An outward-facing video feed on the locomotive also did not record a strike, according to Sumwalt.

The FBI will be looking at a damaged windshield on Amtrak 188. Sumwalt said the windshield was shattered in the derailment, but there's one area, on the lower left side, that they will be focusing on and analyzing the fracture pattern.

What About the Speed?

Whether or not the locomotive was hit by a projectile, it still doesn't explain the train's quick acceleration. The train does not have an automatic throttle — meaning a person must move the throttle to control the speed. Sumwalt said investigators are working to rule out all possibilities including a "mechanical anomaly."

Investigators plan to comb through data from the black box which records movement of the throttle.

A 3D scan was made of the interior and exterior of the train. Investigators plan to reconnect the engine's brake lines and conduct tests in the coming days to determine whether they were operating properly. Initial findings showed the train only decelerated by four miles per hour after the emergency brake was applied.

[NATL] Dramatic Images: Amtrak Train Derails in Philadelphia

Bostian is well-versed in his job, investigators said, and is described by friends as someone who's loved trains since he was a child.

Brad Watts, a longtime friend of Bostian and former NBC News producer, told NBCNews.com he is a good person who cares deeply about his job.

"He knows the responsibility that it takes for a person to get people to and from their destination safely," he said. "Something happened … catastrophic to cause the train to derail."

Watts has not spoken to his friend since the derailment, but has been in touch with his significant other, he said.

"Knowing Brandon myself, Brandon cares a lot about people and I can imagine that it's tearing him up inside," he said.

The NTSB is wrapping up their on-site investigation and planning to head back to Washington, D.C. to continue the probe. Sumwalt did not have a timeline when the investigation would be complete.

Two Trains Hit Before Crash

SEPTA Regional Rail train 769 heading northbound toward Trenton, New Jersey, was hit by something at 9:05 p.m., the transit authority told NBC10 Wednesday. The impact of the unknown object shattered the engineer's windshield and forced the train out of service just south of North Philadelphia station.

About 10 minutes later, Amtrak Acela 2173 traveling southbound on the same line was also hit by a projectile. Passenger Madison Calvert was sitting next to the window that was damaged.

SEPTA/Madison Calvert

“I’m like ‘Oh my God, my window’s shattered,’” he told NBC10 Thursday. A photo he shared showed a large circular fracture in the glass.

Johns Hopkins student Justin Landis, who was also traveling on Amtrak Acela 2172, told NBC News, that an object hit the train, shattering the window. He said the train was near the Philadelphia 30th Street Station around 9:20 p.m. when he was startled by the the noise.

City officials said earlier this week they didn't believe the incidents were connected to the derailment.

In SEPTA radio recordings, obtained by NBC10 Friday, police initially thought someone may have fired a gunshot at the regional rail train. That turned out not to be true, according to SEPTA Police.

A SEPTA spokeswoman said regional rail trains running through the same area have been hit by rocks, thrown by kids or vandals, in the past. They continue to investigate.

Sumwalt said investigators have retrieved video from the SEPTA regional rail train and were listening to radio communication recordings. The regional rail employees will be interviewed by the NTSB.