Metro is investigating a report of a door opening on a Red Line train during Tuesday morning’s commute. However, it’s still unclear exactly what happened aboard the train bound for downtown Washington.

“We did have a report of a Red Line train experiencing a door issue,” said Caroline Lukas, a spokeswoman for the transit agency.

A train heading from Medical Center to Bethesda stopped after an indicator showing that all doors were closed turned off. The train operator inspected the train, but didn’t find any issue with any of the doors, Lukas said.

A traveler tweeted at 8:25 a.m. that a train door opened in the tunnel. However, no customers reported any door problems to the operator, she said.

There were no other reports of train doors opening on Twitter, though one rider tweeted about a Red Line train off-loading following a door malfunction at Medical Center.

The train continued to Bethesda and was removed from service “out of an abundance of caution,” Lukas said.

The doors-closed indicator can turn off any number of ways, she said. For instance, if a customer is leaning on a door and the train shifts, causing the passenger to move slightly, that could open the door just a bit and turn off the indicator.

Metro is still investigating to see what happened.

Earlier this year, doors opened on two Red Line cars carrying travelers between stations. One commuter told the Washington Post that all three sets of doors opened and closed multiple times. Metro investigated the potentially deadly malfunction and found that an electrical short was to blame.