Potomac Electric Power Co. officials said they were still mystified yesterday about the cause of an electrical fire that popped several manhole covers and poured acrid black smoke into the air along Pennsylvania Avenue NW Thursday afternoon, snarling late rush-hour traffic.

D.C. Fire Department sources said that a sparking short circuit in a low-voltage line may have touched off a small fire that grew enough to disrupt several heavy duty 13-kilovolt feeder lines that run through the same conduits.

The incident started at about 5 p.m. at the intersection of 20th Street and Pennsylvania Avenue, and before the fire died out 2 1/2 hours later, it had spread underground halfway to 19th Street.

"We are not conducting a full investigation because it was in a Pepco manhole, not a city manhole, that the fire occurred," said Gwen Smith, a fire department spokeswoman. "This could have been storm related, but we're just not certain." She said that the remnants of Hurricane Bob that blew through the city Thursday afternoon could have contributed to water flooding the conduits, causing a short circuit.

William H. Jones, Pepco's vice president for corporate affairs, said that crews worked throughout Thursday night to ensure that power was available to buildings in the area.

"Our problem, in understanding this fire, remains the fact that an electrical fire alone doesn't usually spread," Jones said. "We just don't have the answers."