Metro got an early warning from riders the day the doors flew open on a Red Line train, and actually closed off one car before doors on a second car on the same train also opened, the agency confirmed Thursday.

But the transit agency kept the rest of the train running, removing the train from service only after the second incident.

A technician met the train at Union Station on Tuesday morning for a separate mechanical issue, according to the Tri-State Oversight Committee, which monitors Metrorail's safety.

While there, riders told the worker doors on one of the rail cars were opening, Metro spokesman Dan Stessel said. The technician closed off one rail car to riders, but it remained part of the train.

Yet shortly after 9 a.m., doors on another car of that same six-car train opened between the Van Ness and Tenleytown stops while the train was in motion. The doors were on the other half of the pair of cars that had been closed off to riders, Tri-State Oversight Committee Chairman Matt Bassett told the Washington Examiner. A rider posted a photo on Twitter, showing riders standing next to the open door. That time, Metro evacuated the train and removed it from service. The blog Unsuck DC Metro first reported that the train had door problems earlier that day.

That riders had reported doors opening before the Tenleytown incident indicates Metro may not have taken enough action initially, putting riders at risk by not closing off both rail cars of that set of cars or removing the whole train from service from the start.

No one was hurt, but doors opening on moving trains is a major safety problem as riders could fall out, exposing them to the electrified third rail and moving trains. It's especially an issue when trains are crowded, as passengers can be packed up against the closed doors.

Metro crews are still testing the train, trying to replicate the doors opening to determine what caused the problem, Stessel said. No one at Metro has been disciplined while the agency investigates, he said.