Justice Department officials said he was arrested at the FBI's training centre in Quantico, Virginia. He had been transferred to the centre, a less sensitive posting, after he fell under suspicion. The department described the arrest as a "major development in a significant criminal case".
The apparent unmasking of a new spy came a month after the arrest of Harold Nicholson, a senior CIA instructor, on charges of passing the names of CIA officials to the Russians in exchange for $180,000 (pounds 110,000). He is the highest CIA official ever to be accused of spying.
Early accounts said the man arrested yesterday had worked at the FBI since 1983, and had specialised in "national security", in other words, counter-intelligence. He had apparently worked for the Russians "for more than five years but less than 10". But it was not clear how serious was the information he had handed over, nor how much he had been paid. Another official said there appeared to be no link between him and Mr Nicholson.
Given the FBI's task of tracking down spies in the US, a turncoat agent would be a huge prize for the intelligence service of Russia or any other power. The last (and only) FBI employee accused of espionage was Richard Miller, jailed for life in 1986 for passing secrets to the Soviet Union.
The comparative speed with with Mr Nicholson was caught was proclaimed as the dawn of a new era of co-operation between the CIA and the FBI.
For Louis Freeh, head of the FBI, and the bureau however, the case is the latest in a string of embarrassments, ranging from its failure to prove sabotage as the cause of the explosion of flight TWA 800 to the botched targeting of Richard Jewell as prime suspect in the Atlanta bombing.Reuse content