Tennent H. Bagley, a former CIA officer who helped a mysterious Soviet spy betray his country in 1962, then tried for a half-century to prove that the defector was actually a Russian double agent, died Feb. 20 at his home in Brussels. He was 88.
The cause was cancer, his son, Andrew, said.
Mr. Bagley's big case caused a war within the CIA and elsewhere in the national security complex for decades over whether the defector he encouraged, Yuri Ivanovich Nosenko, was really a Soviet agent sent to trick the Americans.
Most compelling, as a member of the KGB, the Soviet spy service, Nosenko, who died in 2008, claimed to have handled the case of Lee Harvey Oswald, the presumed assassin of President John Kennedy, during the 2½ years Oswald lived in the Soviet Union after defecting there in 1959.
After the assassination, one conspiracy theory posited that Oswald had been acting as a Soviet agent when he shot the president in Dallas in November 1963.
But Nosenko insisted that the KGB had dismissed any idea of using Oswald for anything -- much less an assassination -- because he was unstable.
The Nosenko case has been examined in a dozen or more books and many articles, some of which were written in-house by the CIA and later declassified.
In 1969, Nosenko was given $80,000, an apology, a new identity and placed on the CIA payroll. Part of his exoneration resulted from the hundreds of investigative leads he provided that turned out to be true.
Mr. Bagley, who once described his career as navigating "a maze of mirrors," never stopped trying to prove that Nosenko was a Soviet plant, writing books and articles that many thought persuasive. One of those books was "Spy Wars: Moles, Mysteries, and Deadly Games" (2007).
Opinion about Nosenko has remained divided.
Tennent Harrington Bagley was born into a naval family on Nov. 11, 1925, in Annapolis, Md. He spent his childhood in France, California, Washington and Hawaii and interrupted his studies at Princeton to serve in the Marine Corps during World War II. After the war, he graduated from the University of Southern California, and later earned a doctorate at the Graduate Institute of International and Development Studies in Geneva.