The late spring afternoon was as beautiful as only Rome can make them. It was Wednesday, 13 May 1981, and the Polish Pope, elected less than three years earlier, was on his way to his regular weekly public audience, being driven in his white - and then unarmoured - popemobile, through a crowd of 20,000 worshippers. It was a routine occasion, yet imbued with the excitement created by this Pontiff, who was already transforming the way the world saw his office.
Suddenly, at 5.17pm, shots rang out. Two bullets struck John Paul II in the stomach. The Pope slumped back, blood staining his white cassock.
For an instant, there was only silence. But disbelief turned to horror, then panic. Cries rang out: "Hanno sparato il Papa! Hanno sparato il Papa! [They've shot the Pope]". A minute later, police grabbed a man fleeing from the square. He was a young Turk named Mehmet Ali Agca - and one of the 20th century's great mysteries had begun.
A letter found in Agca's pocket did little to elucidate matters. "I, Agca, have killed the Pope so that the world may know of the thousands of victims of imperialism," it said. In the event, of course, John Paul II was not killed and, after a five-hour operation, went on to make a full recovery. Yesterday, normality of a sort returned for Mr Agca.
After a quarter of a century behind bars, a greying Mr Agca, now 48, emerged from a jail in Istanbul, a free man at last, in theory. He had been sentenced in Italy to life imprisonment, but at the Pope's instigation, he was released in 2000 as part of a millennial amnesty by the Rome government. Agca was sent back to Turkey to serve the remainder of a 10-year sentence for his part in the murder of a liberal journalist in the late 1970s.
But why did this Turkish petty criminal try to kill the supreme symbol of the Catholic faith? From his note, it seemed as if he had acted alone. But could he have acted alone? In Washington and some other Western capitals, the authorities quickly convinced themselves he had not. This surely was the work of the Soviet KGB, acting through its catspaws in the Darzavna Sigurnost, the intelligence service of Bulgaria, Moscow's most faithful and unquestioning ally in the Eastern Bloc. At first glance, the theory made eminent sense.
The Cold War was at its coldest. Poland, the Pope's homeland, was in turmoil. Millions had turned out to welcome him during his June 1979 visit; led by the Solidarity trade union, the grievances of shipyard workers in Gdansk were turning into full-scale insurrection against the Communist regime imposed by Moscow. The following summer, the Pope held an emotional meeting at the Vatican with Lech Walesa, the Solidarity leader - sealing the Pontiff's commitment to a free, Christian Poland.
By the time of Agca's attempt on his life, the chaos had only deepened. Warsaw Pact forces were conducting exercises on Poland's borders, and a Soviet-led invasion seemed imminent. The murder of the Pope, if the tracks could be suitably muddied, must have been an attractive proposition to Yuri Andropov, the head of the KGB (and mastermind of the Soviet intervention in Hungary in 1956). It would remove the chief spiritual and moral rallying figure for the Polish resistance.
There was supporting circumstantial evidence too. Agca's travels after an escape he made in 1979 had taken him on several occasions to Bulgaria, where he could have been recruited for his lethal mission. A couple of well-documented books, most notably Time of the Assassins by the journalist and acknowledged terrorism specialist Claire Sterling, made the case powerfully for a Bulgarian-Turkish plot.
Alas, despite the best efforts of the Reagan administration, no flesh was ever put on the bones of this "Bulgarian connection". In March last year, a few days before the Pope died, reports circulated in Italy that the Darzavna Sigurnost was about to open its archives - but nothing to date has emerged.
Instead it seems, if anything, that the "Bulgarian connection" was little more than disinformatsya, put about this time by the West to pressurise its adversary in Moscow. In October 1991, a senior CIA analyst on Soviet affairs told a Senate committee that the agency had earlier come up with no hard evidence of Soviet involvement - only for his superiors to alter the report's main judgments and "stack the deck" in favour of Russian complicity. Sections of the report expressing doubts and counter-arguments were erased, and the finished project sent to the White House and the Pentagon, avid to nail the Kremlin. (Any similarities with the CIA's recent handling of evidence for Iraq's supposed WMD programme are, of course, coincidental.)
The ploy, however, worked brilliantly. It reinforced Ronald Reagan's preferred image of the Soviet Union as an "evil empire" (compare the "axis of evil" vilified by the current occupant of the White House), and - some say - distracted attention from the ties between the CIA and unsavoury elements on the Turkish ultra-nationalist right.
Prosecutors in Italy (itself described by some at the time as "the Bulgaria of Nato") vigorously pursued the theory. But in 1986 the trial of three Bulgarians and three Turks allegedly involved in a vast conspiracy to kill the Pope ended with blanket acquittals for lack of evidence. In the courtroom Agca depicted himself not as a latter-day Ramon Mercader, sent by Stalin to assassinate Leon Trotsky in Mexico City in 1940, but as a Messiah, the re-incarnation of Jesus Christ.
Two days after John Paul II's death last year, Agca sent a letter from his Turkish prison asking to be allowed to attend the funeral. "The divine plan has come to its conclusion," he wrote, implying that the assassination attempt was part of God's will. His ramblings only increased the confusion. Was Agca a sane man pretending to be mad - or a holy fool with a veneer of sanity?
The former Italian magistrate Ferdinando Imposimato, the chief prosecutor in the Agca case, is convinced of the former. He insists that the Turk had been nobbled by the Bulgarian and KGB agents in 1983 when he was being held at Rebibbia prison in Rome, and was persuaded to change his story.
"I think the Turkish government should guarantee Agca's security because he knows so many secrets and may be killed," Mr Imposimato said this week. "The best thing would be to keep him in jail."
But events since suggest the latter. In 1983 the Pope visited Agca in his prison cell and forgave him. In 2002, during a visit to Bulgaria, he expressed his conviction that that country was not involved. Agca has repeatedly portrayed himself as a divine agent. More recently, he has claimed to be the fulfilment of the so-called "Third Secret of Fatima", allegedly revealed by the Madonna to three shepherds near the Portuguese town of that name in 1917 - predicting that a pope would be assassinated, part of a global war between Islam and Christianity.
Of that, more in a moment. Of more practical relevance, nothing that has emerged from KGB or Bulgarian archives since the collapse in 1991 of the Soviet Union has indicated that Sofia or Moscow had a hand in events.
Unsatisfying it may be for conspiracy theorists, but the preponderance of evidence suggests that Agca, like Lee Harvey Oswald, acted alone. And just like JFK's assassin, he seems too small for the earth-shattering deed he came within an ace of accomplishing.
If anyone plotted to kill John Paul II, it was almost certainly Turkish extremists, perhaps linked to Ankara's intelligence services, but by no means controlled by them. In that sense, the assassination attempt was a classic case of "blowback" - the unintended consequences of covert action.
At about the same time, the CIA was arming the Afghan mujaheddin in their war with Moscow. A dozen years later, it ended up with Osama bin Laden.
Almost a quarter of a century later, Mehmet Ali Agca the man remains as much of a mystery as that afternoon in St Peter's Square. Back then he was a 23-year-old drug peddler, hoodlum and part-time terrorist who identified with the Grey Wolves, a militant far-right group with links to the drugs trade that frequently clashed with leftists.
In 1979 he had escaped from jail where he was serving a 10-year sentence in connection with the killing of the liberal journalist Abdi Ipekci, who had been investigating links between Turkey's government and organised crime and far-right extremists. Agca went to Europe and North Africa, using at least two expertly forged passports.
He dressed well and never seemed short of money. When he arrived in Palermo, Sicily, on a Tunisian ferry and checked into a hotel on the evening of 13 December 1980, the owner recalled him as "a distinguished, well-mannered person." That day six months later he seemed, according to a news agency report, "a modishly dressed young tourist" in the Roman sunshine - until he pulled out a 9mm pistol and fired it at the Pope.
But if Agca wasn't the pawn of a superpower, was he a figure of far greater contemporary relevance, a harbinger of the war of civilisations, between Christianity and Islam, that some say is now coming to pass? At this point, re-enter the "Fatima connection". The theory is based on the date, 13 May. On that day in 1917, Mary the Mother of God is said to have appeared to the three shepherds near Fatima.
On another 13 May, exactly 64 years later, the Pope was shot - in conformity, it would later be argued, with the "Third Secret". Pope John Paul II, it has been further argued, was especially devoted to Mary, and visited the shrine at Fatima to thank her for interceding to save his life on 13 May 1981, in what he insisted was a divine miracle.
But wait. Is not Fatima merely the Portuguese for Fatma, the name of one of the daughters of the Prophet Mohamed, dating back to when the Moors occupied the country? For Islamic fundamentalists, might not the appearance of the Madonna be a deliberate "provocation" by the Christian infidels, that had to be punished by the killing of a Pope? The name of Agca's mother, it might be added, is Fatma.
Then stir in the tale of Emanuela Orlandi, the teenage daughter of a Vatican employee who was abducted in 1983. Her fate is still unknown. But some believe she was kidnapped to send a message to the Italian and Vatican authorities to keep quiet about the Agca case. In 2001 a skull, which might have been hers, was left at a church close to where she used to live. And the date of this macabre delivery? Naturally, 13 May.
Like the mystery of Agca, the Orlandi case remains unsolved, a footnote to one of the great riddles of the late 20th century. The fate of its protagonist, free after spending more than half his life behind bars, is unclear. Such is the outrage in Turkey at his release that his case may be reviewed. If he remains free, he may have to perform the military service he dodged a couple of decades ago. But the full truth about what happened on that 13 May of 1981 may never be known.
Rupert Cornwell was a correspondent in Rome in 1981Reuse content