Despite a host of CIA measures to keep tabs on employees, and Mr Ames' lavish way of life - which should have raised eyebrows at once - he remained undetected for more than eight years. Arguably the most important Russian spy ever in the US, his career began - if court documents are to be believed - just as the CIA was reeling from the exposure in 1985 of Edward Lee Howard, the last important Soviet agent from its own ranks.
At that time, safeguards were strengthened and access to classified material put under even tighter control. 'Lie detector' tests became more frequent for CIA employees, while all their non-official foreign trips had to be reported.
But Mr Ames slipped through the net. It was years before the discrepancy was noticed between his government salary of dollars 70,000 (pounds 47,000) and a way of life which included the cash purchase of a dollars 540,000 home and credit card spending of dollars 50,000 a year.
Mr Ames' undoing appears to have been a string of failed CIA operations in the former Soviet Union that no coincidence could explain. By 1992 circumstantial suspicions pointed in his direction. As usual, the affair was entrusted to the FBI, and the affidavit filed this week by Special Agent Leslie Wiser makes gripping reading.
Bureau surveillance managed to read traffic on Mr Ames' home computer. Agents copied disks loaded with incriminating evidence. They bugged his house, searched his rubbish and tailed him past sites in Washington where he and his handlers left signals.
One was a post-box on the road travelled by Soviet diplomats between their residential compound and the embassy. Here Mr Ames is alleged to have signified his agreement to a meeting in Bogota in November 1993. In computer documents retrieved by the FBI from Mr Ames' home, the post-box is called 'SS (signal site) Smile.' A second site was another post-box in a quiet Washington neighbourhood, which Mr Ames and his wife visited on 9 September 1993 after attending a parents' night at their five-year-old son Paul's school.
By then however, the Ameses had allegedly been in Moscow's service for eight years. To their neighbours in the Virginia suburb of Arlington they seemed an unassuming couple, devoted to their son. Maria Ames attended part-time classes at Georgetown University. Her husband said he worked for the State Department like his father (who, in fact, was a CIA analyst).
But, says William Rhoads, who lived three houses away, it 'raised eyebrows that they paid cash for the house and simultaneously bought two cars, including a Jaguar'. Mr Ames quelled doubts by implying wealth from his Colombian-born wife's family. The accusation he was a spy came as a bombshell to Mr Rhoads and others.
Even if Mr Ames co-operates, investigators will require months, possibly years, to assess the damage. 'You have to trace all the people he worked with, the information they had access to, the places they worked,' said Robert Gates, the CIA chief in 1991-93.
Conceivably, a full confession could throw light on one lingering mystery - the defection and re-defection in late 1985 of Vitaly Yurchenko, deputy chief of KGB spy operations against the US and, albeit briefly, the CIA's biggest-ever catch from the old Soviet Union. Why Mr Yurchenko fled back to Moscow after tipping off the CIA to Howard's treachery has never been satisfactorily explained. When he defected, the CIA already suspected a mole - and it could be that his mission was to sacrifice Howard to preserve Mr Ames.
According to Vincent Cannistra ro, a retired CIA officer who worked with Mr Ames, he was responsible for the capture and execution of at least two Soviet citizens working for the CIA. As a former head of anti-Soviet counter-intelligence, Mr Ames would have known every detail of the agency's operations in the former Soviet Union.
The extent of any damage done to British interests is unclear. Traditionally, British and US intelligence sharing is great. For example, during his defection, Mr Yurchenko is said to have warned the CIA about the danger to Oleg Gordievski, the London KGB station chief who was about to be revealed as a British agent. With Mr Ames the same process might have taken place - except to Moscow's advantage.