Metro’s 3000-series rail cars are back in service.
The transit agency tweeted Wednesday that it “identified the cause of Sunday’s door malfunction and completed inspections.”
The cars have been returned to service. Metro pulled more than 250 of the cars Tuesday.
Metro said no other service should be impacted.
Metro said it would release more information on the cause but did not indicate when.
A Metro employee saw a video online around 4 p.m. Monday of the open door, triggering an investigation of the cars at the West Falls Church rail yard and a notification to the Washington Metrorail Safety Commission.
It appears no one on the relatively empty car pressed the intercom to tell the operator the door was open while the train was moving, General Manager Paul Wiedefeld said Tuesday, and it is unclear whether the operator was aware of what happened
The investigation looked at a number of potential causes, from mechanical problems to human error. The doors were not in an automatic operation mode.
The decision to pull the trains from service raised concerns because it came just days before Metro shuts down the Franconia-Springfield, Van Dorn Street, Huntington, Eisenhower Avenue, King Street and Braddock Road stations to repair platforms and make other upgrades at the stations.
The summer closure on the Blue and Yellow lines in Virginia will last until September.