NEW YORK - Investigators said Monday there was no evidence that a firearm damaged the windshield of the US train that crashed last week in one of the country's deadliest rail disasters in recent years.
Eight people were killed and more than 200 passengers injured when Amtrak Train 188 derailed in Philadelphia on May 12.
The Federal Bureau of Investigation was called to determine if a projectile had hit the train before it derailed.
But the National Transportation Safety Board announced that the FBI had found "no evidence" of any damage to the train's windshield "that could have been caused by a firearm."
An assistant conductor reported that she heard the train's engineer say it was struck by something shortly before the crash, NTSB member Robert Sumwalt said last week.
The woman also said she heard over her radio that a separate train had been "hit by a rock or shot at," causing that engineer to make an emergency stop.
The NTSB said Monday it has "not ruled out" the possibility that "another object may have struck" the doomed train's windshield.
The entire investigation would take up to a year but additional updates would be announced later this week, the NTSB said on its Twitter account.
Four passengers injured in the accident filed a federal lawsuit Monday seeking damages that could help challenge a legal cap of $200 million.
The four-count lawsuit accuses Amtrak of causing the "tragic and preventable" accident through "reckless operation on the tracks and (an) inexcusable decision to not include a necessary safety system."
The train was traveling at more than 100 miles (160 kilometers) per hour -- twice the speed limit -- when the driver slammed on the emergency brakes just before the crash.
The plaintiffs are New York advertising creative officer Daniel Armyn, Amy Miller from the New Jersey university town of Princeton and Spanish tourists Maria Jesus Redondo Iban and Felicidad Redondo Iban.
They sustained "serious and disabling" injuries "as a result of the careless, reckless and outrageous conduct" of Amtrak, the lawsuit said.