Amtrak Engineer Was Distracted During Deadly Philly Crash, NTSB to Announce

— -- The National Transportation Safety Board is expected to announce tomorrow what led to Amtrak 188's fatal derailment on May 12, 2015.

A source close to the investigation told ABC News that the NTSB has concluded that the engineer at the controls of the Amtrak train that derailed outside of Philadelphia last spring -- killing eight people and injuring more than 200 -- lost awareness of the situation and was likely distracted by radio dispatches about rocks being thrown at trains.

After the train ran off the rails, the engineer in charge of the controls said he barely remembered the moments leading up to the crash, claiming there were "several gaps" in his memory.

The engineer, identified as Brandon Bostian, has reportedly been cooperative with investigators since the crash. He suffered a concussion in the incident.

The NTSB said the train was traveling at 106 mph in an area where the speed is restricted to 50 mph.

The NTSB will hold a board meeting tomorrow in Washington, D.C., to review an investigative report on the crash.