PHILADELPHIA -- The National Transportation Safety Board said forensic experts from the FBI "found no evidence of damage that could have been caused by a firearm" on the windshield of the Amtrak locomotive which derailed in Philadelphia last week.
In a statement, the NTSB said the FBI's examination of Train 188's locomotive was completed Monday.
"The NTSB has not ruled out the possibility that another object may have struck the windshield," the agency said.
An NTSB spokesperson also told CBS News it was far too early to determine the cause of the derailment, which killed eight passengers and left more than 200 people injured. The comments came in response to reports that human error caused the train to derail.
"There is still much investigative activity left to do," the spokesperson said. "The NTSB, and only the NTSB, will make the determination of cause. And when we do that, quite some time from now, it will be publicly announced."
The agency said on Twitter the derailment's probable cause won't be determined until the investigation is complete -- which could take another 12 months.
Brandon Bostian, the engineer who was at the controls, suffered a head injury in the crash. He told the NTSB on Friday that he can't remember anything after leaving the Philadelphia's 30th Street station, the last stop before the derailment, until after the crash.
Last week, an assistant conductor on train 188 told investigators she remembered a radio conversation between Bostian and a regional train operator about their trains being struck by objects.
However, the engine of the regional train spoke with the NTSB over the weekend and did not recall any such conversation, CBS News correspondent Kris Van Cleave reported. A review of dispatch tapes found no communications from Bostian reporting an impact.
Another Amtrak train also reported being struck by something in the same area shortly before the derailment. Philadelphia's Regional Transit Authority says it receives two or three reports each month of objects being thrown at trains.
In the meantime, Amtrak resumed full service Monday for the first time since the crash.
"Crossing over the newly replaced tracks, the train slowed noticeably as we went around the curve where the derailment happened," Van Cleave reported from on board the first train between Philadelphia to New York City early Monday morning.
Passenger Bryan Cummings told CBS News he had no hesitation about taking Amtrak.
"It's one of the safest ways to travel, and you know, just living in fear of another accident isn't the way I want to go about traveling," Cummings said.
Before returning to service, federal regulators required Amtrak to activate technology which can stop a speeding train before it approaches the curve where the accident happened.
The railroad is also adding speed limit signs and reviewing the approved speed for curves through the system.
Also Monday, one of the Amtrak conductors aboard Train 188 has sued the rail carrier. Emilio Fonseca filed the lawsuit Monday in Newark, New Jersey. The complaint accuses Amtrak of "negligence and carelessness" and seeks unspecified damages.
Fonseca's attorney, Bruce Nagel, says his client is still hospitalized after sustaining broken bones and head trauma. Four passengers have also sued Amtrak, which has said it doesn't comment on pending litigation.
Five years ago, federal accident investigators recommended that the government require video cameras in locomotive cabs to record engineers' actions. But it didn't happen.
Train 188 was equipped with a "black box" data recorder and a camera focused on the track ahead. Information from those devices shows that in the last minute before the crash the train accelerated rapidly, reaching 106 miles per hour just before entering a curve where the speed limit was 50.
Maximum braking power was applied in the last few seconds, according to the NTSB, but it was too late.
What investigators would like to know is why the train accelerated. Was it a deliberate act by the engineer? An accident? Or was there some other reason?
It's exactly the kind of circumstance that the requested inward-facing video and sound cameras was supposed to address, according to Jim Hall, who was the board's chairman in the 1990s.
According to Hall, it's not unusual for engineers to be killed in train crashes, or to be seriously injured and not remember details clearly.
"To not have all the investigative tools when people have lost their lives in order to understand what occurred and to prevent it from recurring is a travesty," Hall said. "The black box can tell us what the controls did, but we don't know exactly what the operator did."
Until recently, the Federal Railroad Administration had opposed requiring the cameras, citing concern for the privacy of railroad employees and worrying that the images might be used punitively by railroads. Labor unions representing railroad engineers have also strongly opposed the cameras.
"Installation of cameras will provide the public nothing more than a false sense of security," Dennis Pierce, president of the Brotherhood of Locomotive Engineers and Trainmen, said following a December 2013 commuter train crash in the Bronx, New York, in which it was later determined the engineer had fallen asleep.
"More than a century of research establishes that monitoring workers actually reduces the ability to perform complex tasks, such as operating a train, because of the distractive effect."
The railroad administration and the union didn't immediately respond to questions Monday from the Associated Press.
In the past few months, the railroad administration has told the NTSB that it intends to propose regulations requiring the cameras. However, no regulations have yet been proposed, and it typically takes federal agencies many months, if not years, to move from proposals to final regulations.
The NTSB first recommended requiring audio recordings of sound in locomotive cabs in the late 1990s following a commuter rail crash in Silver Spring, Maryland. None of the operating crew members survived the crash, and the board was unable to determine their actions leading up to the crash. The recommendation was repeated in about a dozen more crash investigations since then.
It was revised to include video cameras with sound five years ago as the board wrapped its investigation into one of the worst train collisions in memory -- a Metrolink commuter train that failed to obey signals and collided head-on with a Union Pacific freight train near Chatsworth, California, in 2008. Twenty-five people were killed, including the Metrolink engineer, and over 100 injured.
The NTSB later determined the engineer was distracted and didn't notice the signals because he had been sending text messages during the trip. But some deduction was involved in that conclusion.
"Clearly the visuals would have revealed what actually happened," said Mark Rosenker, who chaired the NTSB at the time of the Metrolink accident.
Over the past five years, the safety agency has repeated the camera recommendation in accident after accident, including as recently as last November when the board concluded its investigation into a collision between Union Pacific and BNSF Railway freight trains near Chaffee, Missouri, in May 2013.
"The NTSB remains concerned that the Federal Railroad Administration's delayed action... leaves many safety lessons unlearned and further delays improvements for the safety of railroad operations," the board said at the time.