Antonov, who worked as a Rome-based representative for Balkan Airlines, Bulgaria's national airline, was arrested in 1981 by Italian authorities and charged with complicity after the assassination attempt against Pope John Paul II by Turkish national Mehmet Ali Ağca on May 13, 1981. Pope John Paul II was seriously wounded, but survived the shooting.
The case against Antonov fell apart. Italian prosecutors could not prove that the Bulgarian secret service had hired Mehmet Ali Ağca to assassinate the Pope at the behest of the Soviet Union, which feared the Polish Pope's influence in then Communist Eastern Europe. Antonov was acquitted of the charges in 1986 following a two-year trial. The Italian court said that there was not enough evidence to support a conviction.
Antonov returned to Bulgaria following his acquittal. He refused to speak publicly about his time in prison. His mental and physical health rapidly declined  and he spent rest of his life in isolation from others.
Pope John Paul II made his first and only official visit to Bulgaria in 2002. The Pope publicly rejected the allegations that Bulgaria's Communist government had been behind his 1981 attack and never believed in the Bulgarian connection.
- Open Society Archives: Records of the RFE Rome Bureau on Antonov trial (boxes 16-19)
- Los Angeles Times: Sergei Antonov, 59; Bulgarian named in a 1981 plot to kill pope[dead link]
- Time.com Healing Old Wounds