In 1984, U.S. spies monitoring the Soviet press found an alarming piece in a Russian magazine. It wasn’t an expose on officials in the Soviet Union or a worrying account about Cold War attitudes toward the United States. Rather, it was a recipe for coot, a small water bird that’s common in Eastern Europe.

For CIA officials, that meant trouble. They had long had an agreement with a Russian double agent they called TOP HAT—if he wanted to get in touch with them, he’d indicate it by publishing the recipe. Was TOP HAT in danger?

As it turns out, yes. Soon after, America’s most valuable spy, Dmitri Polyakov, fell off the map entirely. For nearly 25 years, the Soviet military intelligence officer had served as the United States’ most trusted resource on the Soviet military, providing reams of intelligence and becoming a legend in the process.

Polyakov’s documents and tips informed U.S. strategy in China during the Cold War and helped the U.S. military determine how to deal with Soviet-era weapons. And Polyakov was credited with keeping the Cold War from boiling over by giving the United States secrets that gave it an inside view of Soviet priorities.

But was Polyakov a double agent…or a triple one who kept the U.S. on an IV drip of false tips and misinformation? And what happened to him after his sudden disappearance?

Polyakov was born in what is now Ukraine in 1921. After serving in World War II, he was recruited by the GRU, the USSR’s military intelligence agency. He wasn’t the type of man anyone would peg as a spy—the son of a bookkeeper, he was an unassuming father who did carpentry projects in his spare time. On the surface, he was a dutiful worker and a reliable GRU asset. But as he rose through the ranks of the agency, following protocol and living a seemingly routine life, he began to work to undermine the USSR itself.

At the time, the GRU had agents all around the world, and was tasked with learning everything possible about American life, priorities, and military assets. The United States did the same thing with the USSR, but had a harder time because of the absolute secrecy that ruled Soviet intelligence.

Until Polyakov offered himself to the CIA as a double agent, that is. At the time, he was stationed at the Soviet Mission to the United Nations in New York. Though Polyakov was fiercely loyal to the USSR, he was increasingly disgusted by what he saw as the corruption and impending failure of Soviet leaders. So he offered his services to the United States.

One CIA officer who worked with Polyakov believed his motivation to help the Americans stemmed from his service in World War II. "He contrasted the horror, the carnage, the things he had fought for, against the duplicity and corruption he saw developing in Moscow," this source told TIME’s Elaine Shannon.

Polyakov considered himself to be “a Russian patriot,” writes author Ronald Kessler. The spy lived modestly and refused to accept large amounts of money for his work. Instead, he insisted on being paid only $3,000 a year. And the money wasn’t delivered in cash. Instead, writes Kessler, Polyakov accepted payment in the form of “Black & Decker power tools, fishing gear, and shotguns.”

It took years for the spy to prove his loyalty to skeptical U.S. intelligence officials. But once he began to pass on information, mistrust turned to glee. Polyakov provided a dizzying amount of material, received by agents during fishing trips (the spy’s fishing rod had a secret chamber for information), tucked into fake stones and flashed via radio transmissions as the spy rode past CIA headquarters on a U.S. Embassy trolley.

The information he passed along proved, among other things, that relations between the USSR and China were becoming increasingly tense. The United States, in turn, exploited those dynamics as it attempted to resume a relationship with China. Polyakov also exposed the espionage of Frank Bossard, a British military officer who was caught selling secrets to the Soviets.

Polyakov was not only fearless—he was well positioned within the Soviet military, where he rose in ranks in the GRU year after year.

“He was absolutely at the top,” said Sandy Grimes, a former CIA officer, in a 1998 interview. Because Polyakov had access to so many kinds of information within the Soviet intelligence machine, said Grimes, he provided unprecedented and unparalleled intelligence.

“Polyakov was a consummate intelligence officer,” Grimes recalled. Motivated by his dislike of Soviet leadership, the “crown jewel” of intelligence officers knew he would pay with his life if his double-cross ever came to the attention of the Soviets. “He knew that if he were caught, he would be sentenced to die.”

In the meantime, Polyakov took advantage of his role as a top officer in the GRU. From his post in the United States, he photographed massive numbers of documents, obtained information face-to-face from dangerous informants, and became a beloved asset to CIA officials, who gave him the freedom to choose his own tactics and even his own missions.

Over time, he passed on a treasure trove of important documents, from Soviet intelligence related to the Vietnam War to monthly Soviet military strategy reports to a list of military technology the Soviets wanted to obtain from the West. Eventually, the information he passed along to the United States filled 25 deep file drawers.

As Polyakov climbed the ranks of the Russian military, he continued providing invaluable information to U.S. intelligence. But in 1980, the double agent was summoned back to Moscow. Then he suddenly retired and disappeared from view entirely.

This unsettled members of the intelligence community, who knew that the Soviets had begun arresting and killing American agents. Though some insisted that Polyakov had simply retired, others worried he had been executed.

Then, in 1990, the Communist Party's official newspaper Pravda published an article that proclaimed Polyakov had been caught in the act of espionage, captured and sentenced to death. Puzzled intelligence experts argued about the purpose of the article—a rare admission that some Soviet spies had worked on behalf of the United States.

“Does he lie in a traitor's grave, as Pravda suggests, or is he a secret hero, quietly retired at the end of a daring career?” speculated intelligence expert Thomas Powers in the Los Angeles Times. “Only one thing about the Polyakov case is now certain: Whoever decided to publish the Pravda story was certainly willing—most probably wanted—to remind the world that the Cold War may be ending, but the intelligence war goes on forever.”

As analysts agonized over the meaning of the report, Polyakov’s U.S. colleagues mourned their friend and cursed the loss of the crucial intelligence he had coordinated. According to Pravda, the spy who had meant so much to the United States had been convicted of treason and executed in 1988.

For years, the U.S. suspected that Aldrich Ames, an American double agent who was convicted of espionage against the United States in 1994, had ratted out Polyakov. But in the early 2000s, officials discovered that Ames wasn’t the only person who had contributed to the agent’s downfall. In 2001, former FBI agent Robert Hanssen was accused of spying for Moscow, and FBI officials learned he had betrayed Polyakov to his Russian bosses.

Hanssen’s admission about Polyakov’s service as a double agent had taken place at least 5 years before Polyakov was charged with espionage, raising questions as to whether the general had been lured back to the Soviet side, perhaps misleading U.S. intelligence in the last years of his service.

So was Polyakov a real asset, or a triple-crossing spy who had sown discord and disinformation in the United States? High-ranking intelligence officials maintain that Polyakov was the real deal. “The guy was legit, absolutely,” an official told the New York Times in 1990. Grimes agrees. “This was a man of tremendous courage,” recalled Grimes. “In the end, we won. The Cold War is over and the Soviet Union was dissolved.”

Former CIA director James Woolsey agreed. “What Gen. Polyakov did for the West didn't just help us win the Cold War,” he told a reporter in 2001, “it kept the Cold War from becoming hot.”