The investigation into last week’s fatal Amtrak train derailment in Philadelphia could turn into a criminal case if FBI analysts find sufficient evidence that deliberate wrongdoing caused the accident. The National Transportation Safety Board, a civilian agency, asked federal experts to assist Monday in efforts to determine if a deliberately launched projectile or bullet caused the crack discovered on Amtrak 188’s windshield.

So far, authorities have been unable to explain why the train was traveling at more than 100 mph in a 50mph zone before the accident, which killed eight passengers and injured more than 200 others. But damage to the windows of two other passing trains on the same night, May 12, coupled with another Amtrak 188 conductor’s claim that she heard engineer Brandon Bostian say "something about his train being struck by something,” has raised suspicions about deliberate wrongdoing, ABC News reports. If suspicions continue to mount, the FBI could launch a criminal investigation.

“In cases of suspected criminal activity, other agencies may participate in the investigation. The Safety Board does not investigate criminal activity; in the past, once it has been established that a transportation tragedy is, in fact, a criminal act, the FBI becomes the lead federal investigative body, with the NTSB providing any requested support,” the NTSB said in an explanation of its investigative process on its website.

If a criminal investigation is announced, it wouldn’t be the first time the NTSB relinquished case control to federal authorities. The U.S. Justice Department spearheaded a criminal investigation on Sept. 11, 2001, after terrorists crashed four hijacked planes into targets in New York and Washington, D.C., the NTSB notes on its website. And the FBI took over the investigation into the 1987 crash of Pacific Southwest Airlines Flight 1771 after evidence revealed a suspect had shot the plane’s pilot.

It’s difficult to predict if the Amtrak 188 investigation will turn into a criminal case until the FBI draws a conclusion on the projectiles. Officials have described Bostian as cooperative and have yet to indicate that he will face charges for his role in the derailment, despite evidence that the train traveled at excessive speed. Both a southbound Amtrak train and a SEPTA train suffered window damage within a few miles from the derailment site the same night.

The SEPTA train’s personnel reportedly warned a dispatcher that a gunshot or a thrown rock may have caused the damage, NTSB official Robert Sumwalt said, according to the Philadelphia Inquirer. The damage to the SEPTA train’s windshield occurred in an area where rocks are regularly thrown at passing trains, a SEPTA representative told BuzzFeed News Friday.

“This is an area where at least once a week vandals throw projectiles at trains. We don’t know who is doing it. Most likely it’s children,” SEPTA spokesperson Jerri Williams said, adding that investigators had not found any evidence to link the damage to the passing SEPTA train to the Amtrak 188 derailment.

Even if federal analysts determine a projectile of some sort was launched at Amtrak 188, it remains unclear how or why that would explain the train’s excessive rate of speed at the time of the derailment. Bostian has submitted to interviews with NTSB officials, but claimed he cannot remember anything that happened after the train left the station in Philadelphia on its way to New York. Despite the other conductor’s claim that Bostian said something about his train being struck by something, an analysis of radio dispatches and an interview with the SEPTA engineer to whom Bostian had purportedly relayed that warning provided no evidence the conversation ever occurred, Sumwalt told CNN.

“Nevertheless, we have the mark on the windshield of the Amtrak train, so we certainly want to trace that lead down,” Sumwalt said.