A Metro train operator has been put on leave and will be disciplined for opening a Green Line train's doors before the train reached a station, according to the transit agency, after the safety mishap exposed riders to the open tracks.

At least one set of doors on the northbound Green Line train opened shortly before arriving at the Greenbelt station at 7:22 p.m. on July 26, according to a rider who documented it with a photograph on his smartphone.

Metro officials investigated after The Washington Examiner started asking questions. They now believe the door opened because of a person, not a mechanical problem such as one that occurred this spring on a Red Line train.

"The operator has been identified and will receive appropriate disciplinary action once the investigation is final," said Metro spokesman Philip Stewart. "We will also issue a safety bulletin to all operators specifically addressing proper and safe door operations."

No one appears to have been hurt during the July incident. But the independent safety oversight group charged with monitoring Metro, the Tri-State Oversight Committee, considers door openings to be "very serious" safety issues, said Chairman James Benton. Riders could be exposed to the electrified third rail or moving trains nearby.

The openings have two primary causes: commanded openings, in which someone is opening the doors, or uncommanded openings, in which something goes wrong mechanically.

After a May 15 incident, in which doors opened on a train twice while moving, Metro inspected all of its 5000-series rail cars for an electrical short believed to have caused the problem. Benton said he believes the agency has since resolved those uncommanded door problems.

"There will be reports of doors opening one inch or less because of the push-back springs," he said. "But they will not open to the point where someone is able to fall out."

The commanded door openings also can be fixed with more than just a safety order, including an engineering solution, which he said Metro officials are working on.

"If we put a spaceship on Mars, we should be able to make the doors not come open," he said.