A tripped circuit breaker in the basement of D.C. police headquarters was responsible for the cutoff of all 911 emergency telephone lines and most police radio channels throughout the city for two hours Thursday night, officials said yesterday.

While workers sought to repair the District's emergency line, the 911 numbers in suburban Virginia and Maryland were overloaded by residents curious to learn if the system was working in their areas, police in various jurisdictions said.

Police Chief Maurice Turner has called for an investigation into who tripped the circuit breaker and why it took two hours to check whether it had been tripped. The circuit breaker is located in a locked room in the basement of police headquarters at 300 Indiana Ave. NW. "If we find out it was purposeful and we find out the individual who did it, we will prosecute," Turner said.

Elbert Ransom, assistant director of the D.C. Department of General Services, said his office is investigating the power failure. He said the problem could be electrical or a human error.

Currently, only the building engineers have a key to the locked room, Turner said. The lock on the door was changed yesterday and an alarm will be placed on the door to guard against unauthorized entry.

The incident also pointed out a weakness in the city's backup system, which was only strong enough to power the lights on the sixth floor, where the communications center is located, and two radio channels. The backup generator is set to generate full power only if there is a disruption of power outside the building, not inside, said Jim Battle, a public information officer with the District police.

He said it is not clear whether the generator can be altered.

Battle said the circuit-breaker switch was not checked immediately because the building engineers and representatives of Pepco "began their probe on the sixth floor the only portion of the building to lose power and worked their way down."

"That's what we'll check first from now on," said Battle.

District police devised a number of ways to deal with the loss of radio contact. Local police stations used runners to carry notes with the assignments and destinations to the patrol cars waiting outside, Battle said. Some officers used hand-held, battery-operated radios.

"Thank God we had no real type of emergency," Battle said.

Suburban police said no emergencies appear to have been affected by the overload of calls they received after reports were broadcast about the District police's power problems.

"It was havoc in here for about two hours," said Col. Fenton Hinson, assistant shift commander of the Prince George's County police communications facility in Landover. "The TV said 911 was broken and everyone started testing their own lines."

Telephone emergency systems in Maryland automatically identify all telephone numbers that call 911, Hinson said, and Prince George's policy is for emergency operators to reach all callers, even those who hang up immediately, to determine if there is an emergency situation. "We had a lot of people just calling in and hanging up," Hinson said. "We're required to call back and check, so it was a lot of extra work."