Investigators say signaling equipment where a deadly Metro transit train crash occurred periodically failed in the days before last week's accident.

Nine people were killed and more than 70 injured June 22 when a moving train slammed into another train stopped on the tracks near the Maryland state line.

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Metro's signaling system is supposed to detect passing trains and transmit speed and stop commands.

But the National Transportation Safety Board said Wednesday that problems began occurring after a piece of equipment was replaced June 17, five days before the crash.

Metro did not immediately respond to a request seeking comment.

The NTSB says the signaling system at the crash site also failed intermittently during tests following the crash.