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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, the BBC reports.

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.