The doors of a Green Line train opened before a train reached the station last week, exposing riders to the electrified third rail and open rail tracks, according to a rider's account and photograph.

And now a train operator or other Metro workers could be disciplined for opening the doors too early -- and failing to report it, according to Metro.

The doors opened about 7:22 p.m. on July 26, as a northbound train was heading for the Greenbelt station.

"The doors were open for at least 15 seconds -- long enough for me to initialize my smartphone's camera and take two photos," rider Aaron Horenstein told The Washington Examiner.

No one appears to have been hurt during the incident. But doors opening on trains not at platforms are a major safety hazard. Riders could be exposed to the electrified third rail or the hazard of falling with moving trains nearby.

It was the fourth case in three months in which riders reported that doors had opened when they weren't supposed to. On May 15, a train's doors opened twice while the train was moving. A rider photographed the second case, which prompted Metro to inspect all of its 5000 Series rail cars. But Metro has said it could not verify the next two cases despite riders' eyewitness accounts.

Now, after The Washington Examiner started asking questions about this latest case and Metro reviewed it, the agency said it believes the doors may have opened inappropriately.

"A preliminary review of available data indicates a possible door opening at Greenbelt station as the train was just off the platform," Metro spokesman Dan Stessel said. "The investigation remains ongoing, and we are analyzing data to determine the exact circumstances."

Metro said it did not have any reports about the incidents, despite Horenstein saying that at least one other rider on the train alerted the operator via the intercom about the problem as it happened.