The Amtrak train that derailed in Philadelphia on Tuesday may have been hit by an object shortly before the crash, a US investigator has said.
He said a conductor had overheard a radio conversation between the Amtrak’s driver and another driver, in which both said their trains had been struck.
But Amtrak driver Brandon Bostian said he could not remember what happened.
The train derailed as it hit a curve while travelling at more than twice the speed limit, killing eight people.
More than 200 people on board the Washington-New York train were wounded.
National Transportation Safety Board (NTSB) member Robert Sumwalt said the assistant conductor had heard Mr Bostian talking by radio with the driver of another train from the Southeastern Pennsylvania Transportation Authority (Septa).
The Septa train driver reportedly said his train – which was in the same area – was hit by a rock or some other projectile.
Mr Bostian said the same had happened to the Amtrak train, according to the assistant conductor.
The investigators said they had found an area of glass which might indicate damage from a flying object, and have called in the FBI for technical assistance.
NTSB officials earlier interviewed Mr Bostian, saying he was “extremely co-operative”, but could not remember what happened.
The 32-year-old driver had called for stricter rail safety.
On various posts to Trainorders.com, Mr Bostian lamented Amtrak’s lack of Positive Train Control, an automatic braking and warning system which was not fully operational on that section of line.
Safety experts have said it could have prevented the crash.
Friends said Mr Bostian talked about trains all the time and always wanted to be a train driver or conductor.
He has worked for Amtrak for nine years and was promoted to train driver in 2010.
On his Facebook page, friends wrote that he is a “great person and a great engineer [driver]”.
Investigators said the train sped up from 70mph (113km/h) to over 100mph in the minute before hitting the sharp bend.
Amtrak boss Joseph Boardman said on Thursday that the agency’s goal is to “fully understand what happened and how we can prevent a similar tragedy from happening in the future”.
The last wrecked coaches were pulled from the scene on Friday and taken away for further examination.
The rail service remains suspended between New York and Philadelphia until at least early next week.