Jeanne Vertefeuille joined the Central Intelligence Agency as a typist in 1954 and then began inching up through the ranks, obtaining postings overseas. By 1986 she had become a midlevel expert on the Soviet Union and counterintelligence. She remained a quiet agency soldier, however — purposefully nondescript and selflessly dedicated. She lived alone and walked to work.

But if she was a gray figure at the agency, Vertefeuille was also a tenacious and effective one, and in October 1986 was asked to lead a task force to investigate the disappearance of Russians whom the CIA had hired to spy against their own country.

Almost eight years later, the investigation led to the unmasking of a CIA employee, Aldrich Ames, as one of the most notorious traitors in U.S. history. He had sold out the Russian agents — at least eight were executed — for millions in cash. His downfall was in no small part owed to Vertefeuille, who brought to the mission a deep knowledge of Soviet spycraft and of her own agency’s workings.

She died Dec. 29 at age 80. In announcing her death, Michael Morell, acting director of the CIA, called Vertefeuille “uniquely suited for the job” and described her as “a true CIA icon.” Some compared her work on the Ames case to that of Connie Sachs, the brilliant researcher for British intelligence in John le Carre’s spy novels.

Sandra Grimes, a CIA veteran who also worked on the case, said Vertefeuille had died of a malignant brain tumor at a nursing home in the Washington area, declining to be more specific.

“Jeanne was one of the most private people you can ever, ever imagine,” she said.

Vertefeuille’s role in the investigation began in 1986 when, as station chief in Gabon, she received a cryptic cable to return to CIA headquarters in Langley, Va. From May through December 1985, she was told, Soviet spies working as American double agents had disappeared at an alarming rate. She was to lead a small task force to investigate, initially composed of two women and two men and later to be joined by Grimes.

The journalist David Wise wrote in his 1995 book “Nightmover: How Aldrich Ames Sold the CIA to the KGB for $4.6 Million” that women had been chosen for the unit because their bosses felt that women would have more patience in combing through records. He also suggested that relatively low-ranking officials like Vertefeuille and the others were selected because the agency was operating on the presumption that no CIA colleague could be a traitor.

“The CIA thought it had picked a minor leaguer,” Wise said of Vertefeuille in an interview with Time magazine, “but she proved she was good enough for the majors. In the end, she got Ames.”

The investigators did not immediately seize on the idea that a Soviet double agent, or “mole,” was operating inside the agency; it seemed just as likely to them that somebody outside the agency was intercepting communications. But there was a mole.

Ames, the son of a CIA officer, had worked as an agency file clerk as a teenager. In September 1983, he was appointed head of counterintelligence in the Soviet division. Two years later, struggling financially, he realized his job gave him something of immense value to Moscow: the names of Soviet agents spying for the United States. He began his treachery by selling two names for $50,000, he later said.

As he fed Moscow names and the spies started vanishing, Ames said, he complained to his Soviet handler. “Why not put a big neon sign over the agency with the word ‘Mole’ written on it?” he recalled saying.

Vertefeuille’s team struggled with the investigation for years. Its members began to be pulled away to other assignments part time. Even after it was discovered, in November 1989, that Ames was living far beyond his means, buying Jaguars and a $540,000 home with no down payment, the hunt stalled.

By early 1991, as Vertefeuille approached the mandatory retirement age of 60, she felt guilty that she had not solved the case, she recalled in “Circle of Treason: A CIA Account of Traitor Aldrich Ames and the Men He Betrayed,” a book she wrote with Grimes that was published last year. She asked to spend her final months of work on the case.

The breakthrough occurred in August 1992, when Grimes discovered that large deposits in Ames’ bank account correlated with his meetings with a Soviet official. The FBI joined the case, finding evidence in Ames’ garbage and computer, and arrested him on Feb. 21, 1994. He pleaded guilty and is serving a life sentence in federal prison.

In a debriefing after his arrest, Ames told his interrogators that when KGB officials had asked for the name of a CIA official whom they might plausibly frame as a mole, he said he gave them Vertefeuille’s name, adding that she was the principal mole hunter.

His admission surprised her.

“At first, I wanted to jump across the table and strangle him,” Vertefeuille said. “But then I started laughing. It really was funny, because he was the one in shackles, not me.”