Catching a 'spy'

Drop site allegedly used by Mr Hanssen on Sunday

The extent of the damage allegedly done to United States intelligence by veteran FBI agent Robert Hanssen is not yet known.

These kinds of cases are the most difficult, sensitive and sophisticated imaginable. I am immensely proud of the men and women of the FBI who conducted this investigation

FBI Director Louis Freeh

But according to bureau director Louis Freeh, information gathered so far suggests a career in espionage for the Soviet Union and Russia spanning the final years and aftermath of the Cold War.

As a 100-page affidavit alleges, Mr Hanssen effectively used his training, expertise and experience as a counterintelligence agent to avoid detection.

The FBI director said Mr Hanssen even kept his identity secret from the Russians. They are believed not to have known his name or his employer until his arrest on Sunday.

Mr Freeh declined to give many details on how the authorities had begun to suspect Mr Hanssen.

'Skilled player'

Louis Freeh praised the government operation to catch Mr Hanssen

But he said Mr Hanssen's training and background as an agent had made him a skilled player, until he came under suspicion in recent months.

The bureau director would not disclose how the agency came to look into Hanssen except to say: "This case was not an accidental case. We did not stumble into it".

According to the affidavit, Mr Hanssen voluntarily became an agent of the KGB in 1985 while he was assigned to the intelligence division of the FBI field office in New York City as supervisor of a foreign counter-intelligence squad.

A central charge is that he disclosed to the KGB the identity of three double agents working for the US.

The agents had been recruited by the government to serve as "agents in place" at the USSR's embassy in Washington, Mr Freeh said.

The three were tried on espionage charges on their return to Moscow. Two were executed, while the third was imprisoned and later released.

Mr Freeh was unable to explain how Mr Hanssen had been able to work for Moscow for more than 15 years while remaining undetected.

FBI agents have began searching Mr Hanssen's residence

But he praised the government operation which, he said, ultimately caught Mr Hanssen red-handed.

An internal intelligence report several months ago first indicated that there was a mole in the FBI.

A joint investigation was launched by the FBI, CIA, the State Department and the Justice Department.

Four-month operation

Mr Hanssen was kept under surveillance for at least four months. His phone was tapped and his house searched in his absence.

The turning point in the investigation came when the FBI was able to secure original Russian documentation of an American spy who appeared to the FBI to be Mr Hanssen.

Alleged methods of protection

Used 'Ramon' as a code name

Routinely checked FBI records

Never met face-to-face with his Russian counterparts

FBI agents also covertly intercepted $50,000 in cash, which Russian intelligence officers are believed to have put into a secret location - a "drop" - for Mr Hanssen to pick up later.

Mr Hanssen is said to have received more than $600,000 in cash altogether, along with diamonds worth $800,000.

He was finally arrested on Sunday at a park in Vienna, Virginia, after allegedly dropping off a package of classified information for retrieval by Russian agents.

Mr Hanssen's activities also have alleged links to other, earlier espionage and national security investigations including the Aldtrich Ames and Felix Bloch cases, according to the affidavit.