After 10 days of investigation into why a rail car door remained open on a moving Metro train, engineers and safety officials say they know the cause: a cup of coffee, spilled on the controls of Car 5190.

The coffee caused an electrical malfunction, which triggered the doors to open twice for a few seconds as the Orange Line train left Metro Center at 2:40 p.m. Aug. 30, heading toward New Carrollton.

"We conducted a thorough investigation of the train's doors, electronics, operating and engineering components . . . and all indications led us to conclude that the coffee that accidentally spilled into the train's console damaged the train's control board, which caused the doors to open," said Fred Goodine, Metro's assistant general manager for safety.

The six-car train stopped at Metro Center, the doors opened, and then the train began to move -- a serious malfunction of the system that is supposed to prevent movement if doors are open. The operator pressed the emergency brake to stop the train, and the doors closed, spokeswoman Lisa Farbstein said. Passengers used the intercom to say they were unable to exit because the doors were closed. When the operator reopened the doors, the train moved again, Farbstein said. The operator pressed a second emergency brake, and the train was taken out of service, she said.

While the investigation was pending, train operators were ordered to use manual controls when running any of the 192 similar rail cars. The order was lifted late yesterday.

Metro's policy against food or drink on the subway system extends to train operators. Transit managers will "follow up and discipline" the coffee drinker, Farbstein said.