On May 16, 1980, a KGB major named Victor Ivanovich Sheymov disappeared from Moscow with his wife and child. Nothing was missing from his apartment, and the few clues left behind suggested that Sheymov and his family might be dead.

By the next day, Sheymov was on his way to the United States, smuggled out by the Central Intelligence Agency in a daring spy operation. He was granted political asylum in the United States and is now a U.S. citizen.

Sheymov, previously unknown to the public, appears to be one of the most unusual Soviet defectors of the Cold War. He worked in the Eighth Chief Directorate, which handles communications intelligence and is probably the most secret and sensitive part of the KGB. At the time he left the Soviet secret service, he had a comprehensive knowledge of all KGB cipher systems and was familiar with the KGB's communications-intercept and code-breaking activities.

He also saw glimpses of the KGB in action around the world that must have intrigued the Americans who debriefed him over many months when he reached this country.

"The peculiar thing about me was that I was in the inner sanctum of the KGB, so I knew the whole system, including the cipher system," Sheymov explained in one of a series of recent interviews about his case.

"He was a major defector who made a highly valuable contribution to our country and national security," said a Bush administration spokesman who is familiar with the Sheymov case, but asked not to be identified. "It is his decision to go public now, and we respect that," the spokesman added. Sheymov's defection has been one of the best-kept secrets in the intelligence business. He said he decided to speak out because he wants to be able to comment on Soviet-American affairs and because, in his words, "No secret lasts forever. This one lasted 10 years, and that's enough."

One of the crucial moments in the gradual disillusionment that led to his decision to defect, Sheymov said, came during a trip to Poland in 1979 when he learned of a KGB cable that he interpreted as indicating that the Soviets might be considering a plan to assassinate Pope John Paul II.

He was meeting with the KGB general who officially headed the Warsaw office when in walked a Col. Solovyev, undercover chief of station. Solovyev looked agitated and said he had an urgent cable from Moscow, Sheymov recalled. "The cable said, 'Obtain all the information possible how to get physically close to the pope,' " Sheymov said. It was signed by the chief of the KGB at the time, Yuri V. Andropov.

"Everyone knew what it meant," Sheymov said. "It meant they wanted to assassinate the pope." He recalls that the KGB general complained to his two colleagues, "If we do that, we'll have to kill them all {the Poles} or get out of here."

Another indication of Andropov's interest in the Polish-born pope was an earlier cable to the Warsaw station, sent shortly after the election of John Paul II. Andropov complained to the Warsaw KGB chief, "How could you possibly allow election of a citizen of a socialist country as pope?" The general, an experienced bureaucrat, confided to Sheymov that he had advised Andropov to check with the KGB office in Rome.

There has previously been speculation that the KGB was involved in the May 1981 attempt by Mehmet Ali Agca to assassinate the pope. That speculation has focused on Agca's alleged "Bulgarian connection." The evidence for this link was murky, and it was undermined by Agca's bizarre behavior during his trial in Rome.

Sheymov said he told his CIA debriefers about Andropov's interest in the pope soon after he arrived in the United States in 1980, months before the May 1981 assassination attempt by Agca. He said he does not know whether the CIA acted on the information.

As he tells it, Sheymov's life story is the stuff of spy novels. He was born May 9, 1946, in Moscow to a well-educated family. His father was an engineer; his mother was a doctor specializing in cardiology. He attended one of Moscow's best high schools, and then entered the elite Moscow State Technical University, where he attended "School M," a special department specializing in missiles and spacecraft.

Sheymov left the university in 1969 to continue his work on missile-guidance systems for a secret enterprise called the Central Scientific Research Institute No. 50 of the Ministry of Defense. "The entire purpose of the institute was research in the military use of space," according to Sheymov. His task involved designing an infrared guidance system for an anti-satellite missile that would be fired from one spacecraft at another. (The Soviets never deployed such a weapon, according to American officials.)

Sheymov left the institute in 1971 and, at the age of 25, was invited to join the KGB. He was assigned to the Eighth Chief Directorate, which at that time handled ciphers and communications intercepts.

In 1974, he was sent to the headquarters of the First Chief Directorate in Yasenevo, just outside Moscow, which oversees all KGB foreign intelligence operations. He served as a communications watch officer, monitoring all incoming KGB message traffic from around the world. Among his responsibilities was helping prepare the daily intelligence summary for members of the Politburo.

The job allowed him to observe the entire range of KGB secret operations around the world. He and the other watch officers kept a log filled with special requests from top KGB officers who wanted to be informed about particular agents and operations.

One series of cables, Sheymov said, discussed whether KGB operatives should try to break the legs of Rudolf Nureyev, the Soviet ballet star who had defected to the West in 1961 and was continuing to make anti-communist statements. Sheymov also recalled hearing office gossip about KGB plans to kill another controversial defector, Yuri Nosenko, who came to the United States in 1964. Some CIA counterintelligence officers contended that Nosenko was a phony defector planted by the KGB.

One murder plot involved putting a poison needle on the seat of Nosenko's car, Sheymov said. Such rumors suggest that Nosenko was a legitimate defector whose departure had put him on the KGB hit list, rather than a plant -- unless one assumes that the gossip was planted.

In 1976 Sheymov was assigned to work on communications security, including such problems as code-breaking and counterespionage. He became a special assignments officer, working on sensitive matters. After discovering how the Chinese had eavesdropped on the Soviet Embassy in Beijing, he was promoted to major -- one of the youngest to reach that rank, he said -- and later assumed broad responsibility for communications security.

A Kremlin power struggle appeared to be looming by late 1979, with Andropov positioning himself to take control after the death of Leonid Brezhnev, which he eventually did in 1982. Sheymov recalled several signs that Andropov was gathering his forces: the cipher staff in the Eighth Directorate, the most trusted part of the KGB, was for the first time given special training in street fighting and use of automatic weapons; dozens of KGB pilots and tank commanders were brought to Moscow for "training" and remained near the city.

Trusted KGB officers were also given new instructions. Rather than the usual formula stressing loyalty to the party, Sheymov recalled, they were told, "You are KGB officers first of all and must fulfill any order you are given."

"The talk in the office was 'Yuri Vladimirovich {Andropov} is getting ready,' " Sheymov remembered.

Sheymov said he decided to leave the Soviet Union in 1980 after increasing disillusionment because the gap between communist rhetoric and reality had become insupportable. "As a product of that society, I was brainwashed from a very young age," he said. "I was a very devoted communist, very patriotic."

But at higher levels of the party and the KGB, "you begin to see independent information available to a few 'trusted' people that contradicts what you were taught before . . . . You are in a position to see what the KGB does. It is supposed to defend the Soviet people, but it doesn't. It works against them and the whole world."

Sheymov decided to defect. "My goal was to inflict as much damage on the communist system as I possibly could," he said.

On his next trip outside the Soviet Union, in early 1980, he slipped away from his escort. Communications personnel never traveled unaccompanied, he said. He walked into a U.S. embassy and told the Marine guard in broken English that he wanted to speak to a representative of U.S. intelligence. Eventually a diplomat emerged.

"I told him I was KGB and which part," Sheymov recalled. "I said, 'I think I can help you, if you can help me. What I need is to get out of the Soviet Union with my family and establish a life in the West.' "

Sheymov and the American agreed on a rendezvous in Moscow, which took place about two months later in a park. The Americans said they were ready to get Sheymov and his family out of the country and grant him political asylum in the United States. About a month later, he was gone.

Sheymov will not discuss any details of the "exfiltration" scheme that was used to get him and his wife and child out of the country.

"The trick wasn't just to leave," he said. "The trick was to make it so the KGB would be led to think something had happened to me, that perhaps I was dead." He did not leave any big clues, which would have been too obvious, but "10,000 little things." The key, he said, was to "preview in your mind the KGB's investigation. If you know how it is done, you have a chance."

One of the few things he took with him was his red KGB identification card, bearing his picture in a major's uniform and the ID number 04035.

After he arrived in the United States, Sheymov spent about a year with CIA debriefing officers. He was awarded the U.S. Intelligence Medal for his contributions, and was resettled in the United States under an assumed name. He became an American citizen in 1985 and now works as a consultant.

According to U.S. intelligence officials, Sheymov's relationship with the CIA was not entirely happy. Sheymov will not discuss the debriefing in any detail. Asked about the CIA's resettlement efforts, which have been criticized by other Soviet defectors, he said, "They definitely have some room for improvement."

Sheymov took his first steps toward going public about 18 months ago, when he offered The Post's Outlook section an article about KGB penetration of the Orthodox Church under the pen name Victor Orlov. That article did appear, as have several others in The Post and elsewhere, under the Orlov byline.

"I would like to put certain events on the record, to put the record straight," Sheymov said in explaining his decision to go public. He said that his wife Olga, a 39-year-old painter, is also happy because she will finally be able to exhibit her work.

Another important reason for surfacing now, Sheymov said, is to let his parents know he is not dead. "I feel that finally I have to say that I am alive and well, and where I am and what I have done."

The former KGB officer said he will hold a news conference today at 11 a.m. at the National Press Club to answer questions about his case.