The greatest peace dividend coming from the collapse of communism may be reaped by history. Hundreds of mysteries that have perplexed Western intelligence services and punctuated the Cold War will get solved as secret files are subjected to sunshine.

The activities of the dreaded Bulgarian secret police have always been one of the darkest corners of espionage and covert action, probably including the attempted assassination of Pope John Paul II. Already coming to light are details of one of the most intriguing spy vs. spy stories of the last couple of decades. And unlike the plot to kill the pope in 1981, in which the Soviets are implicated as directing Bulgarian agents, this attempt at murder was probably a matter of personal pique by a former Bulgarian leader.

The story revolves around the murder of Georgi Markov, a Bulgarian dissident, by a poison-tipped umbrella, the signature style of that country's former secret police. Markov was a popular writer and TV commentator in Bulgaria during the 1960s, and is well-remembered and respected. He was a personal protege of former Bulgarian boss Todor Zhivkov.

Markov was showered with money and special privileges owing to his close friendship with Zhivkov's only daughter. He also enjoyed wide access to state secrets, particularly Communist archives. As he sifted through the state-sponsored sleaze, he apparently found his soul. In 1969, he fled to the West with his brother.

Zhivkov was deeply wounded and felt personally betrayed. But the Bulgarian leader might have let it go if Markov had kept his silence about his former "friends" and country. Instead, he went to work for BBC radio's Bulgarian service and for Radio Free Europe. He was tough on former friends, but Zhivkov kept his cool for several years about Markov's damning broadcasts.

In early 1978, Markov apparently crossed a line: He read on Radio Free Europe chapters from his memoirs that contained excerpts of conversations with Zhivkov and very stinging criticism.

A defector from the Bulgarian secret service, Durzhavna Sigurnost, told us during a trip to that country: "Zhivkov was particularly angry with Markov after these broadcasts. Normally, he tried to bribe his critics with presents and money. Markov was an exception."

Soon Markov began receiving death threats and grew wary of strangers and chance encounters. But even friends thought his paranoia had reached a peak when he fell sick the evening of Sept. 7, 1978, and told a bizarre story. He said he had been returning home from the BBC and was waiting for a bus in a crowd on the Waterloo Bridge. He felt a sudden, sharp pain in his right thigh and turned to see a large man with an umbrella who apologized to him in a thick accent before leaving in a taxi.

Markov contracted a raging fever, and was convinced that he was a casualty of a poison-tipped umbrella. Four days later he was dead. All doubts were erased by the autopsy, which revealed a tiny metal pellet with two holes drilled in it under his thigh skin.

It became a murder investigation. But the actual poison was unknown until a friend of Markov's, Vladimir Kostov, also a Bulgarian defector but living in Paris, heard the story. He reported that a few weeks before he had been at a Metro station and felt a stinging pain in his back. He was examined closely, and the same kind of pellet was found with deadly ricin poison inside -- apparently unreleased thanks to Kostov's heavier clothing.

Scotland Yard then worked overtime to track Markov's killers, but knew that the trail would lead to a brick wall -- or perhaps Iron Curtain. No Western intelligence agency had been able to penetrate Bulgaria's DS, which was shrouded in secrecy, totally loyal to the Communist government and the Soviet KGB and considered the contract hit-men for the satellite services.

With the dawning of a new era, the leaders of Bulgaria have promised Markov's British widow a full investigation to come up with the truth. Bulgarian agents and Scotland Yard are now cooperating, an inconceivable development just two years ago.

Unfortunately, this is one trail that may be too cold. Knowledgeable U.S. and British intelligence sources told us that three Bulgarian generals implicated in the murder were believed to have destroyed files and transferred some to the KGB in Moscow as anti-Communist opposition leaders rose to power in Bulgaria in the last year.

However, transcripts of Markov's broadcasts have been posthumously collected and published in a book in English in Great Britain and the United States titled, "The Truth That Killed." Friends had tried to convince Markov that it would be a mistake to include in his memoirs extensive passages against Zhivkov. Yet it was those very passages that have made him a revered figure to his countrymen, who no longer need to secretly huddle next to radios to hear the truth on BBC and Radio Free Europe.