The State Department said today that it had ordered the expulsion of a Russian diplomat who was apprehended outside the department's headquarters with electronic equipment for monitoring a listening device hidden in a conference room used by senior American diplomats.
The order to expel Stanislav Borisovich Grusev, an employee of the Russian Embassy here, came after a long investigation that also uncovered a tiny sophisticated transmitter on the top floor of the State Department, law enforcement officials said.
Government officials said their investigation had uncovered a ''very aggressive'' Russian intelligence operation that represented a serious penetration of the State Department. The Federal Bureau of Investigation and the department are still trying to determine the extent of the damage to national security from the eavesdropping operation, which may have been running for months before it was detected early this fall.
United States officials said the case marks the first time that the State Department headquarters is known to have been successfully penetrated with listening devices by a foreign intelligence service.
F.B.I. counterintelligence agents detained Mr. Grusev on Wednesday in a public park across the street from the State Department. Mr. Grusev, identified by the federal authorities as an embassy attache, was a technical expert from the S.V.R., or Russian foreign intelligence service, officials said. He was not charged with any crime, and was quickly turned over to Russian officials after he invoked diplomatic immunity.
Russian officials were later directed to remove Mr. Grusev from the United States within 10 days.
Neil Gallagher, a senior F.B.I. official in charge of counterintelligence operations, told reporters at the State Department that investigators first became suspicious of Mr. Grusev almost by accident early last summer, when an F.B.I. surveillance team working on another case noticed him outside the department. The F.B.I. agents recognized him from their surveillance of Russian Embassy employees.
''They made an observation that Stanislav Grusev was standing in the vicinity of the State Department, and it took them as a little bit odd, and they just remembered that very brief observation and came back and developed a surveillance strategy,'' Mr. Gallagher said.
But other officials said the F.B.I. had information before Mr. Grusev was detected that led them to suspect a Russian operation was under way at the State Department.
Over time, F.B.I. agents watched how Mr. Grusev often drove up to the department, got out of his car and walked around in the surrounding streets and parks. Mr. Gallagher said that Mr. Grusev always parked his car -- with diplomatic license plates -- in the same location, and that that action had attracted the attention of surveillance agents.
Agents documented his movements around the State Department, the frequency of his visits and the activity surrounding his car, and concluded that the Russians had planted an electronic eavesdropping device inside the department. After a search lasting several weeks, a transmitter that Mr. Gallagher said was a ''very sophisticated'' and ''sensitive technical'' piece of espionage equipment was found this fall.
''This episode should be a stark reminder to all of us in government that despite the thawing of tensions between competing nations, government facilities and personnel remain a desirable target for foreign intelligence services,'' Assistant Secretary of State David Carpenter told reporters.
Law enforcement officials said the investigation of Mr. Grusev began months before the recent espionage incidents that evoked the cold war practice of retaliatory expulsions. In late November, the Russian authorities expelled an employee at the American Embassy in Moscow after they said they had found her carrying spy paraphernalia, including a device designed to detect signals of electronic surveillance gear.
And just before that, a Navy enlisted man who had worked at the super-secret National Security Agency was arrested and charged with passing secrets about American submarine espionage operations to Moscow.