By Andrew Tangel And Ted Mann 

PHILADELPHIA--The FBI will examine the windshield and front end of an Amtrak train involved in a deadly derailment this week after the train's crew reported that it may have been hit by a projectile, the National Transportation Safety Board said Friday.

NTSB member Robert Sumwalt said the information came as investigators interviewed three of the train's crew members, including engineer Brandon Bostian. Mr. Bostian was "extremely cooperative" but didn't remember anything after passing the North Philadelphia station, Mr. Sumwalt said.

Eight people died and more than 200 people were injured Tuesday when Northeast Regional Train 188 jumped the track. The train was traveling more than twice the speed limit of 50 miles an hour as it approached the sharp curve where it jumped the tracks, investigators have said.

Without Mr. Bostian's insight into the crash, investigators face the tough task of unraveling the mystery of why the train accelerated from more than 70 miles an hour to 106 mph over 65 seconds.

An assistant conductor on the Amtrak train heard through her radio a Southeastern Pennsylvania Transportation Authority commuter train's engineer report that he thought his train "had been hit by a rock or shot at," and heard Mr. Bostian say that his own Amtrak locomotive may have been hit by something as well.

A spokeswoman for the Septa said one of that agency's Trenton line commuter trains was hit by an unidentified projectile around 9:10 p.m. Tuesday night in the vicinity of the North Philadelphia station, which sits on the Northeast Corridor tracks about 2 miles before the site of the derailment. Vandalism of moving trains in the area is a common occurrence, she said.

Two people familiar with the matter said another Amtrak train was reported hit with a projectile around the same time. The derailment occurred about 9:21 p.m.

A spokesman for Philadelphia District Attorney Seth Williams confirmed Friday that the office is investigating the derailment. The office is gathering evidence and working in tandem with the NTSB, spokesman Cameron Kline said, but hasn't yet decided whether to launch a formal investigation.

Investigators have taken a blood sample from Mr. Bostian but haven't revealed test results. Authorities are also expected to examine his cellphone. They also will check inspection records, conduct mechanical tests and interviews with railroad employees.

Mr. Bostian, 32 years old, known by friends as a rail enthusiast who loved his job, was familiar with the busy Northeast Corridor route between Philadelphia and New York City. Mr. Bostian had been driving the route for about three years and didn't work irregular shifts that might lead to exhaustion, one person familiar with the matter said.

But it isn't clear how long Mr. Bostian, who had been driving trains for the national passenger railroad for about five years, had been driving the particular locomotive that was pulling seven passenger cars in the Tuesday train wreck.

The Amtrak Cities Sprinter ACS-64 had been in service for a little over a year, NTSB's Mr. Sumwalt said, adding the agency knew of no problems such as sudden acceleration with the model.

Amtrak has ordered 70 of the Siemens AG-made electric engines, the last of which are expected to be delivered by the end of 2016, and which are prized in part for their improved acceleration, a railroad spokesman said.

The curve where the accident occurred was known to railroad-industry insiders, fueling speculation by some observers that Mr. Bostian may have lost track of where he was on the line. But it isn't clear whether the engineer was directly responsible for the train's acceleration, or a mechanical system or malfunction led to the pickup in speed.

Mr. Ditmeyer said the train's acceleration didn't strike him as out of the ordinary. "Perhaps he was accelerating too soon having thought he had passed this curve," Mr. Ditmeyer said.

An onboard safety system designed to ensure the engineer is alive and alert apparently didn't automatically trigger the train's brakes, people familiar with the matter said. A so-called "alerter" system sounds an alarm to prompt the engineer to push a large red button or other device on the dashboard if the engineer's hands aren't moving the controls for a certain period. "This is all a big mystery," one of these people said.

Mr. Sumwalt said investigators would examine whether the alerter--a high-tech version of what was once called a "dead man's switch"--was working properly.

But Mr. Sumwalt noted the safety board has seen cases where fatigued engineers reflexively respond to the alerter, avoiding an automatic triggering of the brakes. He likened it hitting an alarm clock's snooze button.

"They can be in a sleepy state," Mr. Sumwalt said, and still "satisfy the alerter."

Mark Morales contributed to this article.

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