updated 7/1/2009 8:00:25 PM ET 2009-07-02T00:00:25

Equipment that is supposed to detect stopped trains failed periodically in the days leading up to a deadly Metro transit train crash in Washington, D.C., federal investigators said Wednesday.

Nine people were killed and more than 70 injured June 22 when a train slammed into another train stopped on tracks.

The National Transportation Safety Board said that signal system problems began occurring after a piece of equipment was replaced June 17, five days before the accident. The NTSB says the signaling system at the crash site also failed intermittently during tests after the crash.

Metro General Manager John Catoe said signal system problems went previously unnoticed and were detected in a post-crash review.

The cause of the crash has not been determined.

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