But there has been little mystery about who was behind the 49-year-old dissident journalist's slaying as he made his way to a London bus stop. For more than 20 years, former KGB officials and investigative journalists have said the operation to silence a prominent communist critic was a collaboration of Soviet and Bulgarian secret services.
Post-communist Bulgarian news media have long pointed to indications of high-level involvement in Markov's death, including the fact that the attack occurred on Sept. 7, the birthday of Bulgarian Communist leader Todor Zhivkov.
"The murder was supposed to serve as some sort of a gift to Zhivkov," the Sofia online news agency Novinite.com commented Thursday after the officially unsolved Markov case was closed.
In "Kill the Wanderer," a 2005 book recounting his yearslong investigation of the Markov case, Bulgarian journalist Hristo Hristov detailed the findings of his review of nearly 100 once-secret files of the State Security Services. He concluded that Soviet KGB agents hatched the plot to kill Markov in cooperation with a Bulgarian security agent codenamed "Piccadilly."
Markov died in a London hospital on Sept. 11, 1978, four days after the poison pellet was stabbed into his leg. The Bulgarian investigation was closed Thursday, a day after the country's statute of limitations on murder was reached.
In the years after his 1969 defection, Markov was one of the harshest and most influential critics of the communist-ruled Soviet bloc. His weekly broadcasts on Radio Free Europe and the
Bulgarian chief prosecutor Sotir Tsatsarov was quoted by the Associated Press in Sofia as saying his office was ready to help British authorities with the case in the event of new evidence or developments.