Metro is investigating another possible train door opening over open tracks after an incident on the Red Line Tuesday morning.

A six-car train heading toward D.C. stopped at Medical Center about 8:17 a.m. after the train operator noticed an indicator had gone off, said Metro spokeswoman Cathy Asato.

The operator walked through the train but saw no problems with any of the doors -- and no one on the train told the operator about seeing a door open inappropriately, she said.

The train was offloaded and taken out of service, causing delays along the line for riders during the peak period.

The transit system is still investigating, but Asato said the agency had not received any reports from riders about doors opening save for a single tweet on Twitter.

It was the latest case of possible door openings while trains weren't at a platform, a serious safety concern.

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.

Metro said it couldn't confirm two subsequent rider reports of openings.

But a July 26 Green Line case, photographed by a rider and first reported in The Washington Examiner, is causing a train operator to be disciplined for opening the doors too early.