The suggestion was raised by an assistant conductor, who told transportation officials that she heard the Amtrak engineer who was driving the train and the engineer from a nearby regional train talking just before the derailment.

The assistant conductor recalled hearing the regional train driver say his train had been hit by an object and then Brandon Bostian, the Amtrak engineer, say his train had been struck, too.

Robert Sumwalt, the official with the National Transportation Safety Board (NTSB) who is leading the investigation, also said that investigators had found concentric circles on the lower left corner of the glass of the Amtrak train - suggesting it had been hit before the crash shattered the glass.

Tuesday's derailment killed eight people and injured about 200 more.

The train sped up to to twice the speed limit when it was supposed to be slowing down as it entered a sharp bend in the tracks on its way from Washington to New York.

The idea of an object hitting the train raised the possibility that the engineer might have been distracted, panicked or even wounded in the moments before the train left the rails.

Mr Bostian, the 32-year-old engineer, however, said he did not recall anything in the few minutes before the crash when he was interviewed for the first time on Friday.

Bostian was "extremely cooperative", said Mr Sumwalt.

The FBI has been called in to investigate the Amtrak's windshield, and investigators will also look at the regional train mentioned by the assistant conductor.

The Philadelphia Inquirer and other media outlets reported that a third train, an Amtrak Acela, had also been hit by an unidentified projectile that cracked a window on one of the cars, citing an account of a passenger.

Investigators have been looking at why the Amtrak train that crashed hit a speed of about 106mph (170kph) in a zone where the maximum speed is 50mph (80kph) just before it flew off the tracks.

It was the deadliest crash on US railways in nearly six years.