The Amtrak train that derailed Tuesday in Philadelphia may have been hit by some kind of projectile, the National Transportation Safety Board said Friday.
Board member Robert Sumwalt said the FBI has been asked to look into the possibility in relation to "damage on the left lower portion of the Amtrak windshield."
The NTSB is acting on a report from an assistant conductor who said she heard the Amtrak train engineer, Brandon Bostian, talking by radio to the engineer of a regional train. The regional train engineer said his train had "been hit by a rock or shot at."
The assistant conductor said she thought she heard Bostian say the Amtrak train had also been hit.
But when questioned by authorities for the first time Friday, Bostian said he had no memory of anything that happened in the moments leading up to the train derailment, which killed eight people and injured more than 200 others.
Earlier, Bostian's lawyer said the 32-year-old engineer had suffered a concussion in the accident and could not remember what had happened just before it occurred.
The possibility of a projectile does not explain why the train was traveling at more than twice the posted speed limit before it crashed.
The NTSB said the Amtrak Northeast Regional train was rounding a curve at 170 kilometers an hour. The posted speed limit is 80 kilometers an hour.
Sumwalt said Thursday that the train sped up in the last minute or so before the wreck, accelerating from 110 kilometers per hour.
He said it was not clear whether the speed was increased manually. So far, he said, investigators have found no problems with the track or the signals.
Philadelphia Mayor Michael Nutter told reporters Thursday, "I do not think that any commonsense person would think that it was in any way OK" for the train to be traveling at the speed recorded on the train's black box, which collected key information during the planned trip from Washington to New York.