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The engineer of the Amtrak train that ran off the rails outside Philadelphia in May 2015, killing eight people, was likely distracted by radio dispatches, a source close to the investigation told NBC News on Monday.

In this aerial photo taken on May 13, 2015, emergency personnel work at the scene of a deadly train wreck in Philadelphia. Patrick Semansky / AP

The engineer at the controls of the Amtrak 188 told investigators in February that he could barely recall the moments leading up the nighttime crash. Instead, he claimed to have a "dream-like" memory of his locomotive going too fast around a curve, and hitting the brakes as he realized the train was going to tip over.

Related: Amtrak Engineer Recalls 'Dream-Like' Memory Before Deadly Philadelphia Crash

Another train's engineer had reported that his windshield had been hit by something — possibly a gunshot.

Investigators a year ago theorized that engineer Brandon Bostian was distracted by those reports, may have also been hit by a rock at about the the same time, lost situational awareness and failed to slow his train.

Eight people were killed and 200 were injured when the train derailed.

The National Transportation Safety Board has scheduled a hearing Tuesday in Washington, D.C., to consider its final report. It's possible the probable cause could change somewhat before the meeting ends.