The origin of a large flash of light and a huge electric arc that hovered over the Rhode Island Avenue Metro station early Sunday, prompting hundreds of calls to the police and fire departments, has transit system officials baffled, a spokeswoman said yesterday.

The flash, which occurred about 12:05 a.m. after the station was closed, was caused by "some kind of an electrical fault. . . apparently an electrical short somewhere in the system," said Metro spokeswoman Marilyn Dicus. She said the origin has not been determined, but there apparently was no damage as a result of the incident.

Dicus said that one track on the Red Line between Union Station and Rhode Island Avenue was closed during normal operating hours on Sunday while Metro officials investigated. She said full operation was resumed when the cause of the flash could not be determined and officials concluded that the line was safe.

According to a D.C. firefighter who went to the scene Sunday morning, smoke was rising from part of the track, but no fire was discovered. He said the station attendant "was scared to death. One side of the tracks lit up with a huge arc, then the other side."

"A Metro police officer said he thought the whole place was on fire," the firefighter said.

Police and fire officials said their 911 emergency lines were flooded with calls from citizens who had seen a large flash of light. Both police and fire officials said the flash was so bright that a plane traveling at 35,000 feet over the District called the tower at National Airport and said "it looked like all of Northeast was on fire."

An airport spokesman said there was no record of the radio call, but that that "it could have happened."

Dicus said the incident was still under investigation.