A 3000-series rail car. Metro officials have pulled all of the transit agency's 3000-series rail cars from service while they investigate why a set of doors on one of the trains opened while it was moving Sunday. (Bonnie Jo Mount/The Washington Post)

Metro said Tuesday that nearly 300 of its rail cars will remain out of service until the agency can determine what caused a set of doors on an Orange Line train to open on its own shortly after the train left the Dunn Loring station on Sunday.

The incident, which involved the agency’s 3000-series rail cars, happened around 1 p.m. after the train left the station, and the incident was captured on video by a rider. Metro officials said they learned of the video Monday. The trains were pulled from service overnight Monday, and on Tuesday morning, the transit agency announced that it had pulled all 284 cars from the series — roughly 23 percent of the agency’s 1,200-car fleet — from service.

At a midafternoon news conference Tuesday, Metro General Manager Paul J. Wiedefeld said officials don’t know what caused the train’s doors to open. He said the cars will remain sidelined until officials can determine a cause and come up with a fix.

“When we don’t know the answer to something, we take these steps,” he said.

In addition to Metro officials, the Washington Metrorail Safety Commission, which is responsible for safety oversight of the transit agency, has launched its own investigation into the incident.

In a tweet, WMSC officials said the agency had been in “close contact with Metro overnight” on the 3000-series rail cars and that it had dispatched a rail vehicle specialist to West Falls Church rail yard, where the cars involved are being stored.

Wiedefeld said that the operator should be have been alerted that the train’s doors were open, but it’s not clear whether such an alert was triggered. The train operator has been placed on leave, which is standard procedure, but Metro officials have not yet interviewed the person. Wiedefeld said the train’s doors remained open only for a few seconds.

Removing the cars from services meant fewer eight-car trains and more six-car trains for the Tuesday morning commute. Officials also adjusted where trains turned back on the Red Line to make another 32 rail cars available.

Even so, Wiedefeld said the morning commute went “very well” and that riders did not report any significant delays. He added that Red Line trains will no longer be turned back at Grosvenor station.

“This is what safety culture looks like,” Wiedefeld said. “We received info, escalated it and acted properly to mitigate the risk.”

He said the impact on service should be minimal this week. Coincidentally, the upcoming closure of six Blue and Yellow line stations, which starts this weekend, means fewer rail cars will be needed.

This isn’t the first time Metro has had problems with doors opening while a train is moving.

In 2015, the agency’s 4000-series cars had to be pulled from service for a door problem. A year later, the 4000 series was again pulled from service after engineers discovered a glitch that posed a collision risk. The series has since been retired.

Wiedefeld said there have been no previous issues with doors opening while the train is in motion with the 3000-series.

Metro plans to eventually replace the 3000-series with the next-generation 8000-series rail cars. The agency has said eventually 85 percent of its fleet will consist of new rail cars.

The 3000-series went into service in the late 1980s.

This story has been updated. An earlier version had an incorrect percentage for the number the pulled rail cars represented in Metro’s fleet. This version has been corrected.