Jeanne Vertefeuille: CIA officer who unmasked the spy Aldrich Ames

She was, at first glance, the classic little old lady. Jeanne Vertefeuille, brought up as an only child, never married. She lived alone in a modest apartment and invariably walked to work. She might have been a librarian, a clerk, or a quiet pillar of her local church community. In fact she led the team that unmasked the most damaging mole in the history of the CIA.

Where espionage is concerned life has a habit of imitating art. In John Le Carre's Tinker, Tailor, Soldier, Spy, Connie Sachs was the unsung researcher who helped catch the fictional Soviet mole, Gerald. Almost two decades after the book appeared, Vertefeuille, a real-life counterintelligence analyst at the CIA, was in charge of the STF, the Special Task Force that in 1994 caught Aldrich Ames, the senior agency official who for nine years had been selling its innermost secrets to Moscow.

Vertefeuille applied to the CIA after studying German and French at university. The Agency took her on as a typist and she rose through the ranks, serving in Ethiopia, Finland and the Netherlands. But by the 1970s she was back at headquarters in Langley in the Washington suburbs, immersed in her true calling, unpicking the workings of Soviet intelligence.

The CIA had its computers and datebases, but like Sachs at Le Carre's fictional Circus, Vertefeuille was its institutional memory on all things KGB. As head of counterintelligence research at the agency's Soviet division, wrote David Wise in Nightmover, his 1995 account of the Ames case, "she could follow the gossamer threads... if a KGB colonel had appeared in Copenhagen under one name and turned up in New Delhi a decade later with another, she would sort it out."

She toiled quietly and unobtrusively, before being posted to Gabon as station chief in 1984. As Wise noted, Libreville might have been just a one-person station in a faraway country; but in an era when the CIA was even more male-dominated than today and female station chiefs an extreme rarity, the appointment was unusual recognition of her ability.

But within two years Vertefeuille was summoned back to Washington for a far more important job. From the autumn of 1985, CIA agents in the Soviet Union simply vanished – a dozen of them in little more than six months. Something was terribly wrong – and the top-secret STF unit, consisting of Vertefeuille and just four others, was set up to discover precisely what.

The operation took more than seven years. False leads and red herrings abounded, and there were other potential explanations for the debacle. The KGB might have cracked the CIA's codes, or it might be a case of bugging (a theory reinforced by the discovery in early 1987 that the new US embassy building in Moscow was riddled with listening devices.) Few at the agency wanted to believe the third possibility, that there was a human traitor among them. But a traitor there was.

Aldrich Ames was the son of a CIA officer but his own career as a spy had been undistinguished, not least because of his heavy drinking. Divorce from his first wife had added financial troubles to his professional ones. Then, in September 1983 and for reasons still unclear, he was placed in charge of counterintelligence in the agency's Soviet Division. The job gave him access to every detail of CIA operations in the Soviet Union – and a new source of income beckoned. From mid-1985 Ames began to sell those details, complete with the names of CIA assets in the country, to the Russians.

Ames worked for the Russians until his arrest on 21 February 1994. Over the years he had received some $4.6m from the KGB; in return he betrayed, according to Wise, at three dozen agents, at least 10 of whom were executed, as well as hundreds of operations.

Ames was high on the list of possible moles from the start, given his access to the relevant material and a lifestyle far more lavish than that offered by an annual salary of barely $70,000 (£43,000). But only in 1992 did the group make the key breakthrough, linking the dates of large cash deposits into Ames' bank account with those of meetings with a Soviet embassy officer.

By this point Vertefeuille was reaching the compulsory retirement age of 60, but she was placed on contract and the investigation continued uninterrupted. Mole-hunter and mole, the little grey-haired lady and her quarry now resigned to his fate, came face to face at his first debriefing, the day after Ames was sentenced to life in jail. As the session ended, Ames delivered Vertefeuille one last shock: in late 1985, as the KGB was rolling up the CIA's networks, he said, he had given the KGB her name, as a person they might frame should it become obvious a mole was at work.

"At first I wanted to jump across the table and strangle him," she wrote afterwards in a book on the case, Circle of Treason, that she co-authored with a colleague in the STF, Sandra Grimes. "But then I started laughing; he was the one in shackles, not me."

Jeanne Ruth Vertefeuille, CIA officer: born New Haven, Connecticut 23 December 1932; died 29 December 2012.