"The FBI has completed its examination of the windshield of the Amtrak #188 locomotive and has found no evidence of damage that could have been caused by a firearm. The NTSB has not ruled out the possibility that another object may have struck the windshield. Additional updates on the investigation will be issued later this week," the NTSB said in a statement.
The NTSB added that its investigation into the cause of the accident is expected to last up to 12 months.
During an earlier interview on ABC News' "This Week," NTSB lead investigator Robert Sumwalt said there was no communication between the derailed Amtrak train engineer to the dispatch center to suggest that a projectile had hit the ill-fated train, which crashed in Philedelphia on the night of May 12.
“We interviewed the dispatchers and 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 News' George Stephanopoulos.
This morning, Amtrak resumed service between Philadelphia and New York City, six days after the derailment, which left eight people dead and more than 200 injured.
The train, which accelerated before the crash, was traveling over 100 mph at the time, well over the speed limit.