PHILADELPHIA (Reuters) - The Amtrak train that derailed in Philadelphia and a separate commuter train in the vicinity may have been hit by projectiles of some kind shortly before the wreck, a U.S. transportation safety official said on Friday, after investigators interviewed members of the Amtrak crew.

But the Amtrak engineer said he had no memory of anything that happened in the moments leading up to the crash when questioned for the first time about Tuesday night's wreck that killed eight people and injured more than 200 others, said Robert Sumwalt, a member of the National Transportation Safety Board (NTSB).

An assistant conductor told NTSB investigators on Friday that she heard the engineer, 32-year-old Brandon Bostian, talking by radio with the driver of another train from the Southeastern Pennsylvania Transportation Authority (SEPTA). The other driver reported that his windshield had been cracked by a projectile that he believed was either fired from a gun or thrown at the train.

According to the conductor's account, Bostian replied that he believed his New York-bound Amtrak train had been similarly struck after pulling out of its previous stop, Sumwalt said.

It was moments later that the Amtrak train barreled into a curve at more than 100 miles per hour (160 km per hour), twice the speed limit, in the city's Port Richmond neighborhood along the Delaware River.

Sumwalt said investigators still have no explanation for why the train was going as fast as it was, and why it had accelerated from 70 mph to 100-plus mph in the 65 seconds before the crash, as was shown on video footage taken by a camera mounted on the locomotive.

The engineer had slammed on the emergency breaking system seconds before the wreck, investigators said.

Sumwalt said on Friday that Bostian, who has spoken with investigators with his lawyer present and was cooperative, told them he had no recollection of doing that, or of anything else from the time the train had departed from its previous stop.

Experts said the train's speed in the moments before the crash raised several questions: Could a technical glitch have caused the locomotive to speed up so rapidly? Would it take a deliberate action by the engineer? Or could human error, a medical issue, or some other factor like clumsiness explain the sudden burst of speed?

Sumwalt said the train, as designed, can only be accelerated by manual control, but the NTSB would examine whether a mechanical malfunction could have caused the train to speed up on its own.

He said Bostian reported to investigators that he had experienced some technical problems on his way south to Washington from New York earlier that day. He did not elaborate.

(This version of the story corrects last name of engineer to Bostian, not Bostion)

(Additional reporting by Ellen Wulfhorst and Barbara Goldberg in New York, and David Morgan and Mark Hosenball in Washington; Writing by Frank McGurty and Steve Gorman; Editing by Jeffrey Benkoe and Lisa Shumaker)