Investigators are still trying to determine the cause of this week's Amtrak derailment, which killed eight people and left 200 injured. Now, officials are looking into whether the train was struck by a projectile before hurtling over 100 miles per hour around a 50-mile-per-hour curve.

An assistant conductor reportedly told investigators with the National Transportation Safety Board that on the night of the May 12th derailment, she heard the train's engineer, Brandon Bostian, tell another engineer over a radio transmission that his train had been struck by a projectile, after the other engineer mentioned it had happened to his train as well. "Right after she recalled hearing this conversation between her engineer and the Septa engineer, she said she felt a rumbling, and her train leaned over and her car went over on its side," safety board official Robert I. Sumwalt said at a press conference yesterday.

Bostian, who claims he has no recollection of the fatal derailment, told investigators he did not recall any projectiles hitting the train. But according to the Times, it's very common for objects to hit trains—conductors call it getting "rocked"—and though generally it's harmless, officials are looking into whether this could have played a part in Tuesday's crash.

Bostian has reportedly been "extremely" cooperative with investigators, despite his memory lapse. He had only been on the route for a few weeks, and had reportedly only had a short rest break after driving an Acela train from New York to Washington earlier in the day, thanks to lengthy delays and electronic issues, and it's possible he was exhausted and "frazzled" by that route. Investigators say the train sped up before reaching the curve, located near Philadelphia; Bostian pulled the emergency brake just moments before the train derailed.

CBS 2 has video showing the cars falling off the rails:

The FBI is also investigating the cause of Tuesday's derailment, and they will examine a part of the train believed to show damage—potentially from a projectile—to see if an outside object could have contributed to the incident.