A part of the track circuit that lost contact with a Metro train moments before it slammed into an idling train last week had been replaced five days earlier and “periodically lost its ability to detect trains” after the repair, federal investigators said Wednesday.

Records reviewed by the National Transportation Safety Board revealed that Metro engineers replaced part of the track circuit June 17, five days before the crash between the Takoma and Fort Totten stations that killed nine and injured more than 70 riders. It was the worst accident in Metrorail’s 33-year history.