Sergei Antonov, the Bulgarian accused of plotting the failed assassination bid against late Pope John Paul II on behalf of the Soviet Union, has died of natural causes, officials said on Wednesday.

Antonov, 59, was found dead in his apartment in downtown Sofia, the interior ministry said. It did not reveal the cause of the death, but doctors said he had probably died two days earlier.

A former representative of Bulgaria's national airline in Rome, Antonov was accused, but later acquitted, of working as an agent for Bulgaria's communist-era secret services, which were closely tied to the Soviet KGB, and planning the May 13, 1981 attack.

He stood trial with two other Bulgarians and three Turks for the failed assassination bid, in which Turkish gunman Mehmet Ali Agca shot the late Pontiff and was arrested minutes later.

Before his conviction of attempted murder, Agca fingered Antonov as a co-conspirator.

But prosecutors failed to prove the Bulgarian secret service had hired Agca to kill the Pope on behalf of the Kremlin, which feared John Paul's influence in events that eventually led to the collapse of communism in eastern Europe 1989.

An Italian court acquitted Antonov in 1986 after a two-year trial, saying there was insufficient evidence for a conviction.

But Antonov's health and mental condition deteriorated after his trial and he spent the later years of his life in isolation.

During a visit to Bulgaria in 2002, John Paul rejected allegations that Bulgaria's former communist government was involved in the assassination attempt and said he had never believed in the so-called "Bulgarian connection".

Last year, a report by an Italian parliamentary commission established to investigate the case said the leaders of the former Soviet Union were behind it. Both Russia and Bulgaria have condemned the report.