A worker at Metro's downtown headquarters ignored alarms at 1 a.m. yesterday that indicated that fire sprinklers had activated in the Mount Vernon Square Station, allowing water to flood the station for the next 31/2 hours and forcing its closure to morning commuters.

The fire sprinkler alarms, like fire alarms, are designed to sound when sensors detect fire in a station.

A Metro employee in the operations control center saw the alarm flashing on a console and heard a corresponding shrill noise but turned it off and failed to notify anyone that anything was amiss at the station, said Steven A. Feil, Metro's chief operating officer for rail.

"This is very, very serious," said Feil, adding that he has not taken disciplinary action because he is investigating the incident. "The seriousness suggests we need to take this to the highest level possible."

The operations control center is often referred to as the brain of the subway -- it is staffed with controllers and supervisors who track every movement on the rail lines and are supposed to react to emergencies and ensure the safe operations of the trains and stations. About a dozen Metro workers were on duty at the center when the alarm went off.

Feil said last night that he had not interviewed the crew to determine who was at fault.

The operations control center has been criticized for mismanaging late-night subway service after the Oct. 10 Redskins game. Train controllers opted to run just one train an hour on several lines, inconveniencing thousands of football fans who sat on idling trains as the night turned to morning.

Deputy General Manager James Gallagher said yesterday that he had taken "severe" disciplinary measures against those responsible for the poor service after the Redskins game, but he refused to elaborate or say if that meant any employee had been suspended, demoted or fired.

Feil suggested that the recent incidents are isolated and do not indicate a systemic problem in the operations control center. He declined to say whether he intended to make personnel changes at the center.

"I don't know why this is happening, but, no, it's not a systemic problem," Feil said. "It's a good system and it has some problems but it's not beyond being able to run it. . . . I'm going to make sure the people who are charged to run the control center are doing just that and are doing it to the highest standards."

Yesterday, the sprinklers inside Mount Vernon Square Station were tripped at 1 a.m. after a sudden drop in electrical power at the station related to maintenance work being performed by a Pepco crew, Metro spokeswoman Lisa Farbstein said. Sensors interpreted the power loss as a sign of a fire and automatically activated the sprinklers, which are behind the escalators that lead from the street entrance to the mezzanine level.

Water on the mezzanine was ankle-deep by 4:30 a.m., when an operator on a train passing through the station reported that water was dripping from the mezzanine. The station manager arrived a short time later to get the station ready for the day and discovered the sprinklers running and the mezzanine flooded.

Metro officials could not estimate how many gallons of water had poured from the sprinklers. No structural damage or equipment problems were reported as a result of the flooding.

"We couldn't open the station because we didn't want our customers walking in water," Metro spokesman Steven Taubenkibel said. On a typical weekday during the morning rush, 1,376 people enter and exit that station, which serves the Yellow and Green lines.

Metro widely broadcast the station closure and operated seven shuttle buses to transport 84 passengers between the Shaw-Howard University and Gallery Place stations. It took maintenance workers more than two hours of mopping to remove the water. The station opened for service at 7:15 a.m.

Michael LaJuene of Reston talks to Fairfax County Supervisor Catherine M. Hudgins (D-Hunter Mill), who accompanied him on his commute as part of Disabilities Awareness Day. Story, Page B5.