Dr. Bagley, the son and brother of Navy admirals, joined the fledgling Central Intelligence Agency in 1950. An intellectual fluent in several languages, he rose quickly, ascending by the 1960s to serve as deputy chief of the Soviet bloc division, and was specifically tasked with countering the activities of the KGB.
At the time, the agency’s counterintelligence efforts came under the direction of James J. Angleton, the CIA official who became a divisive figure for his passionate pursuit of Soviet “moles” or infiltrators, who he believed were undermining the agency from within. In 1974, under CIA Director William E. Colby, Angleton surrendered his post.
Dr. Bagley embarked on what would become his most noted work in 1962, when, at a Geneva safe house, he met KGB agent Yuri Ivanovich Nosenko. Nosenko would become one of the most controversial figures in the history of U.S counterintelligence, and Dr. Bagley was described as his chief handler.
In time, according to published reports, Nosenko disclosed to his U.S. interlocutors key information about Soviet infiltration of Western embassies and about his country’s intelligence-gathering practices.
Regarded as more impressive were Nosenko’s later revelations about Lee Harvey Oswald, whom Nosenko said he had interviewed during Oswald’s stay in the Soviet Union in the years before the 1963 assassination of President John F. Kennedy. Nosenko told CIA officers that Oswald had no connection with the KGB — a significant assertion at a time when many officials feared that the assassination could be linked to the Soviets.
During the long period of Nosenko’s debriefing, some in the CIA — including Dr. Bagley and Angleton — noted significant inconsistencies in the information that he provided and concluded that he was not a real defector but rather a “plant” employed by the Soviets.
When Nosenko came to the United States in 1964, he was subjected to what The Washington Post called a “three-year harsh detention and hostile interrogation” including “body searches, verbal taunts, revolting food and denial of such basics as toothpaste and reading materials.” Dr. Bagley maintained that the interrogation did not include torture.
He prepared what were described as 900 pages of material about Nosenko. The report noted that Nosenko never “broke” under interrogation, The Post reported. Despite this, the report offered what was described as extensive circumstantial evidence that he was indeed a plant.
Nosenko passed numerous lie-detector tests, and the CIA determined in 1969 that he had been a genuine defector. The agency later employed him as a consultant. He died in 2008 in an undisclosed location in the United States.
In 1967, Dr. Bagley became CIA station chief in Brussels. He held the post until he stepped down in 1972. After a second career as a consultant, he pursued a third career, writing books.
In 1990, with former Soviet agent Peter Deriabin, he published “KGB: Masters of the Soviet Union.” In 2007, he published the memoir “Spy Wars: Moles, Mysteries, and Deadly Games.”
“It is a stunner,” David Ignatius wrote in The Post of the 2007 book. “It’s impossible to read this book without developing doubts about Nosenko’s bona fides. Many readers will conclude that Angleton was right all along — that Nosenko was a phony, sent by the KGB to deceive a gullible CIA.”
In the New York Times, reviewer Evan Thomas wrote of “Spy Wars” that “though many intelligence old-timers will not be persuaded, Bagley offers a provocative new look at one of the great unresolved mysteries of the Cold War.”
A third book, “Spymaster: Startling Cold War Revelations of a Soviet KGB Chief,” was published in 2013 and recounted the experiences of Sergey Kondrashev. Like Dr. Bagley’s previous books, it included extensive interviews with his former foes, some of whom he traveled to Eastern Europe to meet. Former Wall Street Journal reporter Frederick Kempe accompanied Dr. Bagley on some of those meetings.
“It’s sort of like two soccer coaches of championship teams getting together and recounting their matches,” he recalled in an interview.
Tennent Harrington Bagley — his mother began calling him “Pete” when he was a child — was born on Nov. 11, 1925, in Annapolis. As a young man, he accompanied his father on naval assignments around the United States and around the world.
After Marine Corps service in World War II, Pete Bagley received a bachelor’s degree in political science from the University of Southern California in 1947 and, later, a doctorate in political science from the Graduate Institute in Geneva. In the early years of his CIA career, he served in Vienna, where he married Maria Lonyay, his wife of 58 years.
Besides his wife, who lives in Brussels, survivors include three children, Andrew Bagley of Columbia, Md., Christina Bagley Rocca of Arlington and Patricia Bagley of Reston; a brother; and five grandchildren.
Dr. Bagley once described the essential difficulty of counterintelligence. “It takes a mole,” he told the New York Times, “to catch a mole.”