The FBI found no evidence of a firearm causing damage to the Amtrak train that derailed in Philadelphia Tuesday, according to the NTSB.
NTSB officials made the announcement Monday afternoon — but they have not ruled out the possibility that another object may have struck the windshield of the Amtrak locomotive, according to a statement on Twitter.
The actual cause of the accident will be determined at the end of NTSB's investigation, which is expected to last up to 12 months, officials said.
The announcements were made a day after NTSB member Robert Sumwalt downplayed the idea that a gunshot caused damaged to the windshield of the train shortly before it flew off the tracks, killing eight people and injuring more than 200 others.
"I've seen the fracture pattern; it looks like something about the size of a grapefruit, if you will, and it did not even penetrate the entire windshield," Sumwalt said during an appearance on CBS’s “Face the Nation.”
Officials said an assistant conductor on the derailed train said she heard the Amtrak engineer talking with a regional train engineer and both said their trains had been hit by objects. But Sumwalt said the regional train engineer recalls no such conversation, and investigators had listened to the dispatch tape and heard no communications from the Amtrak engineer to the dispatch center to say that something had struck the train.
"But, nevertheless, we do have this mark on the windshield of the Amtrak train, so we certainly want to trace that lead down," he told CNN's "State of the Union."
Sumwalt acknowledged, however, in an interview on Fox News Sunday that train engines are routinely struck by various projectiles without catastrophic consequences.
Investigators remain focused on the acceleration of the train as it approached the curve, finally reaching 106 mph as it entered the 50-mph stretch, and only managing to slow down slightly before the crash.
"The only way that an operable train can accelerate would be if the engineer pushed the throttle forward. And ... the event recorder does record throttle movement. We will be looking at that to see if that corresponds to the increase in the speed of the train," Sumwalt told CNN.
The Amtrak engineer, who was among those injured in the crash, has told authorities that he does not recall anything in the few minutes before it happened.