NEW YORK (AP) -- A U.S. official briefed by investigators says an Amtrak engineer was distracted by radio transmissions before his train derailed in Philadelphia last year, killing eight people.

The official was not authorized to comment publicly because of the ongoing probe and spoke to The Associated Press on the condition of anonymity.

The National Transportation Safety Board is scheduled to meet Tuesday to detail the probable cause of last year's fatal derailment. The cause won't be determined officially until the board's vote at the conclusion of that meeting.

It's unclear what radio transmissions investigators are citing.

Engineer Brandon Bostian told investigators after the accident that he recalled radio transmissions that night from a local commuter train in the area reporting that its windshield had been busted by rocks. He told investigators that he was concerned for his safety.

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