The investigation into the Amtrak train derailment in Philadelphia this week took a new turn Friday as investigators began examining damage to the locomotive's windshield to see if it had been struck by an object or shot at shortly before the crash.

The National Transportation Safety Board said it had interviewed an assistant conductor on the train, who said she thought she remembered the train's engineer saying something over the radio about the locomotive being struck.

Asst. conductor believed she heard radio transmission from her engineer that his locomotive was also being struck. #Amtrak — NTSB (@NTSB) May 15, 2015

She also overheard a radio conversation between the Amtrak engineer and the engineer of a nearby train belonging to SEPTA, the regional train operator in Philadelphia. The other train's engineer reported that his train "had been either hit by a rock or shot at," according to the conductor's account. The other train's windshield had been broken, according to this account.

Asst. Conductor heard radio transmission from a SEPTA train ahead that had been placed into emergency and stopped. #Amtrak — NTSB (@NTSB) May 15, 2015

Late Friday, SEPTA spokeswoman Jerri Williams confirmed that the commuter train had been hit with a projectile before the Amrak train derailed, according to CBS News. She stressed that the incident did not appear to be related to the accident that occurred some 20 minutes later.

The NTSB has asked the FBI for assistance in examining "damage to the lefthand lower portion of the Amtrak windshield," said NTSB official Robert Sumwalt.

NTSB has asked FBI for technical assistance in examining the front of the locomotive. #Amtrak — NTSB (@NTSB) May 15, 2015

The engineer himself, identified as Brandon Bostian, 32, has no recollection of the crash because he suffered a concussion, his lawyer has said.

Sumwalt said Friday that Bostian had spoken with investigators and was "extremely cooperative." But Bostian told the NTSB that he could not remember anything that happened after the train passed the North Philadelphia station.

Last-minute acceleration

As officials try to determine whether the train had been hit by anything before the crash, they are still puzzling out why the train accelerated in the minute before it entered the curve where it flew off the tracks.

Image: BOB AL-GREENE/MASHABLE

On Thursday, Sumwalt detailed the train's speed in the seconds before it derailed:

65 seconds before the crash: Above 70 mph

43 seconds before the crash: Above 80 mph

31 seconds before the crash: Above 90 mph

16 seconds before the crash: Above 100mph

The speed limit on the curve is 50 mph; the speed limit on the straight track just prior to the curve is 70 mph. The train reached 106 mph, more than double the speed limit of the curved track, just moments before the crash. The engineer applied the emergency brakes just three seconds before the "black box" recording stopped, presumably at the time of the crash. The last reading of the train speed was 102 mph.

Emergency personnel work the scene of a deadly train wreck, Tuesday, May 12, 2015, in Philadelphia.

The assistant conductor who described the radio conversations said that before the crew departed Washington, they conducted a safety briefing where they reviewed the speed restrictions along the route, according to Sumwalt.

At the controls

Sumwalt did not name Bostian, but he confirmed details that match the previous reports about him: He began working at Amtrak as a conductor in 2006, then took on a job as an engineer in 2010. He has been based in New York City since 2012.

Amtrak assistant conductor BRANDON BOSTIAN stands by as SANDRA PALMER of University City says goodbye to her boyfriend, CLYDE SIMPSON, as he leaves for work in Chicago at the Amtrak station. Bostian was the engineer of the Amtrak train that derailed in Philadelphia. Image: Corbis/Huy Richard Mach/TNS/ZUMA Wire

On Tuesday, Bostian was running his typical route, which he works five days a week: From New York to Washington and back again.

Bostian has a "very good working knowledge of the territory," Sumwalt said.

Friends have said that Bostian grew up loving trains. In his previous posts on Internet train forums, he complained about lax safety at railroads, saying that new safety systems could prevent crashes caused by operator error or fatigue.

Officials have said that a speed-limiting system called Positive Train Control could have prevented Tuesday's derailment, but Amtrak had not yet switched it on in the Philadelphia area.

Updated at 8 p.m. ET to reflect SEPTA confirmation that commuter train was hit with a projectile before the Amtrak crash.