Alexander Feklisov, who died Friday at 93, was the spymaster who oversaw Julius Rosenberg and Klaus Fuchs as they stole secrets that helped the Soviet Union develop nuclear weapons during the Cold War.

A spokesman for Russia’s current foreign agent service told the press: “Feklisov made an important contribution to the activity of Russia’s foreign intelligence network in New York on nuclear issues. He conducted serious missions related to the procurement of secret scientific and technical information, including in the area of electronics, radio location, and jet aircraft technology.”

Feklisov’s role began in 1941. From that date to 1946, he was a KGB “Rezident,” or station chief, working at the Russian Consulate in New York City. It was there that his superior, Anatoly Yatskov (alias Yakovlev) gave him the task of recruiting people to engage in military and industrial espionage against America. His most famous agent had already been recruited by his predecessor, but Feklisov became the man who ran the ring put together by Julius Rosenberg, who with his wife, Ethel, was executed for conspiracy to commit espionage in 1954.

Feklisov became, as he called his memoir, “The Man Behind the Rosenbergs” (2001). Befriending Julius Rosenberg, he became not only his controller, but his father figure, his friend, and his comrade in arms. When Rosenberg was arrested, Feklisov sank into deep despair.

Decades later, in 1997, he went public with the full story. The reason, he made clear, was that he wanted Julius Rosenberg to be regarded as a hero for his valiant effort on behalf of the Soviet Union’s great anti-fascist cause. Rosenberg, he wrote, was an “unreconstructed idealist,” a “partisan” who “did not want to betray his Russian comrades.”

In working for the Soviets, Feklisov wrote, Rosenberg “helped the USSR fight the Nazis” so he could “build a peaceful future for his children.” He did not explain how Rosenberg, who enlisted as an agent during the years of the Nazi-Soviet Pact, was doing his service because of his opposition to fascism. He was a hero because Rosenberg “brought our common victory closer to becoming reality.”

Feklisov’s account provided missing links about the extent of the damage done by the Rosenberg spy network. He testified about the major successes. The most important piece of information that the Rosenberg gave the Soviets was an actual proximity fuse detonator. The fuse allows a shell to explode at a short distance from an airborne target, guaranteeing a direct hit. It also corrects the path of an explosive charge toward a plane, a precursor of missile homing devices. The Soviets used one to shoot down Major Francis Gary Powers’s U–2 plane in 1960, thereby derailing the Eisenhower-Khrushchev summit.

Other members of the ring, Joel Barr, Al Sarant and William Perl, provided equally important data such as the SCR584, a device that determines the speed and trajectory of V–2 rockets, that was part of some 600 pages of texts and drawing photographed by the ring members in one evening. Perl, a scientist working for NACA, the predecessor of NASA, gave Feklisov advanced aeronautical data about high-performance military jet aircraft. Through this material, the Soviets build the MIG fighter jets used against the Americans in the Korean War.

Feklisov also provided more information about Ethel Rosenberg’s brother, David Greenglass, who had given Julius Rosenberg a sketch of the A-bomb that he had obtained working at Los Alamos during the war. He also provided evidence that co-defendant Morton Sobell was another spy who gave Feklisov major military data. Mr. Sobell has continued to deny that he spied.

In 1946, Feklisov left America for Moscow. Soon he was transferred to London, where he continued his work in espionage. In 1947, he became the deputy chief of intelligence for the KGB in London. In that capacity, he made contact with Klaus Fuchs, the top atom spy who had moved to Britain and provided Feklisov with new information pertaining to the hydrogen bomb. The material Fuchs gave to Feklisov allowed the top Soviet A-bomb scientist, Igor Kurchatov, to use American models and science to build the Soviet nuclear arsenal.

Feklisov’s major work took place during the World War II era and the early Cold War. But he would play one other part in history. He returned to America in 1960 and became chief of Soviet intelligence in Washington, D.C.

In that capacity, he became involved in the Cuban Missile Crisis during John F. Kennedy’s presidency. He worked with an ABC News correspondent, John Scali, with whom he stood as a go-between between the Soviet government and the Kennedy administration.

Feklisov confirmed that the Soviet SS4 missiles in Cuba were missiles with an offensive capability and not defensive ones, as some Cold War revisionists argued. Meeting many times with Scali, who passed his information to the administration, Feklisov was able to get replies from the White House directly to Nikita Khrushchev, bypassing the Soviet ambassador, Andrei Gromyko. As the intermediary between Moscow and Washington, Feklisov was the mechanism that allowed Kennedy to defuse the situation and get Khrushchev to dismantle the missiles. It was, perhaps, his finest hour.

Feklisov retired from the KGB in 1986. He lived to see the collapse of the communist system he supported and spied for. “In spite of all,” he wrote, “I don’t have the feeling that I worked for the wrong cause.” He died an unreconstructed communist.

Mr. Radosh, a contributing editor of The New York Sun, is co-author of “The Rosenberg File” and author of the introduction to the American edition of Feklisov’s memoir, “The Man Behind the Rosenbergs.”