Four Fairfax County 911 dispatchers and call-takers resigned after an internal police department investigation produced allegations that they had used illegal drugs while off duty, county police said yesterday.

The three-month investigation by the police internal affairs section, described as "exhaustive," was begun after information was received from sources outside the police department, spokesman Warren Carmichael said. The civilian dispatchers -- who were not identified -- are alleged to have used "amphetamines and other" drugs while off duty, Carmichael said.

"The bottom line is there was absolutely no use of any drugs by any of these people while on duty," said Carmichael. "Service to the public was not affected in any way." Police department employes are prohibited from using illegal drugs either on or off duty.

He said the former dispatchers and call-takers, who worked at the county's Emergency Operations Center, resigned in June and July and have not been charged with a criminal offense.

Carmichael confirmed the resignations after the department was questioned by a television reporter. Personnel matters are not ordinarily made public.

Carmichael said "nothing in their performance on duty would have aroused suspicion" of any illegal drug use on the part of the employes. He said there was no evidence of any drug use among other dispatchers at the center.

The county has about 100 civilian dispatchers and call-takers who field police and fire emergency and nonemergency calls over the 911 line, he said.

The county had problems when it switched to a new computer-aided dispatch system in June at the communications center, currently at the county's new Pine Ridge Facility. There were complaints of delays in answering 911 calls and rerouting them to the proper dispatcher.

Officials attributed the problems to defects in the software and the difficulty of learning a new system. They said dispatchers were initially working double duty by monitoring the old system to check the performance of the new.

Carmichael emphasized that the drug use by the employes was not related to performance at the new center.