Emergency workers inspect the engine of a derailed Amtrak train in Philadelphia, May 15, 2015. Investigators are still trying to determine what caused a crack in the train's windshield. Reuters

Investigators trying to determine the cause of last week’s deadly derailment of an Amtrak train near Philadelphia say a bullet wasn’t the cause of a crack in the train’s windshield, but they don’t know what else might have led to the damage, according to the local ABC affiliate. The NTSB tweeted Monday afternoon that the FBI found “no evidence” that a firearm caused damage to the windshield, but that it hasn’t ruled out the possibility that some other object hit the windshield:

The investigation into the derailment in Philadelphia included a 1 ½-hour interview with Brandon Bostian, the engineer of Amtrak Train 188. Robert Sumwalt, an NTSB board member, said Bostian made no mention of an object striking the windshield moments before the train, which was going twice the speed limit into a turn, crashed.

"We listened to the dispatch tape, and we heard no communications at all from the Amtrak engineer to the dispatch center to say that something had struck his train,” Sumwalt told ABC’s “This Week” on Sunday, according to Reuters.

The NTSB investigation into the derailment is expected to last up to 12 months, the agency tweeted, letting the public know it may take that long for them to determine the probable cause of the crash.

After several days of limited service followig the crash, Amtrak service along the Northeast Corridor, which stretches from Washington, D.C., to Boston, was restored on Monday. The incident spurred fights among federal lawmakers on Amtrak funding, with Republicans accusing Democrats of playing politics over the crash, and pointed out that America’s other infrastructure is aging.

The train was going more than 100 mph at the time of the crash and was navigating a curve that had a 50mph speed limit. It’s unclear what was to blame for the high speed.