A component 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.

The National Transportation Safety Board reported that an “impedance bond,” a critical component within a track circuit, was replaced on June 17, five days before the June 22 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.