Soon after shooting and gravely wounding Pope John Paul II in St. Peter's Square, Mehmet Ali Agca dropped a hint to Italian magistrates that it would be worthwhile investigating a "Bulgarian connection" to the assassination attempt.

The clue was buried in the middle of a rambling "confession" in which the Turkish gunman depicted the attempt on the pope's life as an act of humanism aimed at stirring the conscience of the world. Describing his travels around Europe, he said he had spent between 40 and 50 days in Bulgaria in the summer of 1980.

It was in the Bulgarian capital, Sofia, Agca went on, that he had been supplied the 9-mm Browning pistol with which he had shot the Polish-born pontiff, as well as the false Turkish passport he was carrying on May 13, 1981, the day of the assassination attempt. He had bought the gun from a Syrian journalism student called "Mohammed," he said. The passport had been supplied to him by a well-connected Turkish businessman named "Omer Marsal."

Agca volunteered "Marsal's" telephone number in the West German city of Munich and the information that he was involved in the smuggling of cigarettes, drugs and occasionally arms. "Marsal," he added, had put him in touch with a mysterious Bulgarian called "Mustafaeof" who later instructed him to investigate the possibility of assassinating the leaders of Tunisia and Malta.

Agca's assertions about how he had acquired the passport and the gun apparently were phony, and both "Mustafaeof" and "Mohammed" have disappeared from later versions of his testimony. But "Marsal" was a real person -- the only person, in fact, whom the Italian police managed to track down on the basis of Agca's first testimony. His actual name was Omer Mersan.

Questioned by West German police on May 21, Mersan was able to confirm that he had met Agca in Sofia. The figure of Mersan also provided apparent evidence of a link between Agca and the smuggling ring known as the "Turkish mafia," which operated out of Bulgaria with the alleged connivance of the Bulgarian authorities.

The first Italian magistrate to interrogate Agca, Judge Domenico Sica, has since told The Washington Post that the pope's would-be assassin gave the impression of having carefully worked out beforehand what his initial testimony would be in the event of his arrest.

"He was calm and confident. He seemed to have a patter all prepared, setting out his arguments. After a few hours, when I tried to start getting specific, he shut up," Sica said.

The hard information that Agca did reveal about Mersan and his stay in Sofia was to become a key element in the early conspiracy theories linking the attempt on the life of the Polish-born pontiff to Bulgaria and the Kremlin. It was not until about 18 months later, in the fall of 1982, that Agca himself made the sensational claim that he had been hired directly by the Bulgarian secret service to shoot the pope.

Much important information about the Italian investigation into Agca's claims is still secret -- filed away among 25,000 pages of evidence that will not become available to the public until the Italian judiciary issues its expected decision to hold a new trial on conspiracy charges. There could well be fresh disclosures. The following represents only a preliminary reconstruction of the complex story of how Agca came to point the finger at Moscow. Turkish Rightist Denies Link to Shooting

One of the first people to accuse the Kremlin of complicity is himself likely to go on trial as a principal suspect in the case. He is Musa Serdar Celebi, 28, the leader of a Frankfurt-based federation of Turkish rightists accused by the Italian prosecutor of providing logistical assistance to Agca and his friends following a military crackdown in Turkey in September 1980.

Eight days after Agca's arrest in St. Peter's Square, Celebi summoned a press conference to condemn the attack on the pope and insist that his organization had nothing to do with it. The real motive, he suggested, was provided by the social upheavals in the pope's native Poland and the rise of the independent trade union Solidarity.

"The assassination is the work of the Soviets, who are deeply worried about developments in Poland," Celebi declared.

When Agca appeared in court in July 1981, he discouraged further investigation by insisting that he had acted alone in attempting to kill the pope and by waiving his right of appeal. Asserting that he should have been handed over to the Vatican for trial, he declared that he would start a hunger strike in December.

The otherwise inexplicable timing of this protest action has been interpreted by most investigators as a signal to his accomplices that he expected to be set free within five months -- the time it had taken to arrange an escape from prison for him in Turkey following his arrest in June 1979 for the murder of prominent newspaper editor Abdi Ipekci.

On July 22, 1981, Agca was convicted on the basis of his initial confession and sentenced to life imprisonment. For a brief period, the case appeared to be closed. But Agca's assertions that he had acted alone did not satisfy the presiding judge, Severino Santiapichi.

Reasoning that the pope's would-be assassin was neither a loner nor mentally unbalanced, Santiapichi issued a statement on Sept. 24 explaining his verdict. Agca, he wrote, was merely the visible point of a "deep conspiracy . . . orchestrated by secret forces, carefully planned and directed down to the smallest detail."

Less than two months later, Judge Ilario Martella was appointed to open a new investigation. Already well known in Italy as the magistrate responsible for uncovering a multimillion-dollar bribery scandal involving the American aircraft company Lockheed and Italian politicians, Martella had a reputation of honesty, a passion for detail and a marked dislike for personal publicity.

According to former defense minister Lelio Lagorio, the first signs that Agca might be prepared to break his self-imposed silence came during a meeting with two Italian secret service agents in his cell at Ascoli Piceno prison on Dec. 29, 1981. This meeting has caused some controversy in Italy, as it took place at a time when Agca was still subject to a regime of rigid isolation. Suggestions in the Italian press that the agents might have tried to encourage the Turk to give evidence against Bulgaria in return for a lighter prison sentence have, however, been specifically ruled out in a report by the public prosecutor.

The defense minister was later to tell parliament that Martella rejected a request for a second meeting with Agca by Italian and Turkish counterintelligence agents in April 1982.

In any event, it was not until May 1982 -- a year after his capture in St. Peter's Square and five months after his interview with the secret service agents -- that Agca began to talk freely to Martella. At this stage, he did not directly implicate the Bulgarian secret service. Instead, during a week of continuous interrogation, he accused the "godfathers" of the Turkish mafia of having been behind the assassination attempt.

The two names he mentioned in particular were Abuzer Ugurlu and Bekir Celenk.

Ugurlu, Agca said, had assisted in his escape from Turkey to Bulgaria and the provision of two false passports in the names of "Yoginder Singh" and "Faruk Ozgun." Celenk's role had been to offer, via a middleman whom Agca refused to name, the sum of 3 million West German marks (then about $1.2 million) for shooting the pope.

Details of the new confession, which ran to 312 pages, were published in the Turkish newspaper Milliyet -- Abdi Ipekci's old paper -- in the middle of July. It was hardly a coincidence that the leak appeared there. Milliyet's efforts to reconstruct the Ipekci murder and Agca's past gave the paper's reporters privileged access to Italian investigators, who were eager to tap their knowledge.

Even though Agca himself still did not point a finger directly at Sofia, the possibility of a "Bulgarian connection" already had become the object of intense interest by the spring of 1982.

Parallel to Martella's investigation, another magistrate, Ferdinando Imposimato, had been pursuing a separate inquiry into reports of a Bulgarian spying ring in Rome involving an Italian trade union official, an extreme left-wing terrorist and several members of the Bulgarian Embassy. Acting on information supplied by the trade union official, Imposimato ordered a confiscation of passenger lists at the offices of the Bulgarian state airline Balkanair in April 1982. He also interrogated Agca.

Italian counterintelligence, meanwhile, was drawing up a photo album of possible Bulgarian suspects. Lagorio, then defense minister, was later to tell Parliament that this album was handed over to the magistrates in June 1982. It was made available to both Imposimato and Martella.

During the summer of 1982, allegations of a direct "Bulgarian connection" to the papal plot also began surfacing in the mass media. August saw the publication of an article in the Reader's Digest by Claire Sterling, a Rome-based journalist, concluding that the Bulgarians had masterminded the assassination attempt on behalf of the Kremlin.

On Sept. 21, NBC television carried a similar report, citing as a possible motive a letter that it said had been sent by the pope to the then Soviet leader, Leonid Brezhnev, at the height of the Solidarity crisis in Poland. According to NBC, the pope had threatened to respond to a possible Soviet invasion of his homeland by resigning "the crown of St. Peter" and fighting alongside his fellow Poles. Agca's Access to Outside News Reports

At the very time he was talking to Italian judges, Agca appeared to develop an almost obsessive interest in what was being said about him in the outside world. There is convincing evidence, for example, that he had access to Italian press reports about the NBC "white paper."

On Sept. 23, two days after the hour-long NBC program was broadcast in the United States, Agca wrote a letter to the Vatican secretary of state, Cardinal Agostino Casaroli, complaining of "inadequate security conditions" in prison and expressing fears for his safety.

Using his rudimentary English, Agca wrote: "I see, I read, I understand that there are some peoples in Vatican that they want to kill me. They are indicating my death in every chance."

The next day, Sept. 24, he sent a similar letter -- this time in Italian -- to one of the participants in the NBC broadcast, Cardinal Silvio Oddi, complaining that he had been threatened and insulted by priests and bishops. "Finally Cardinal Silvio Oddi you have done the same things (in the Panorama weekly an Italian magazine and to the American journalists)," Agca wrote.

Insisting that he had not entirely given up hope in the Vatican, Agca added enigmatically: "We will see what will happen in the future."

Further indication that Agca had access to news media reports speculating about a "Bulgarian connection" to the papal plot is provided by the fact that he sent a duplicate copy of the Casaroli letter to Francesco Mazzola, the head of a parliamentary watchdog committee overseeing the secret services at the time of the assassination attempt on the pope. Mazzola had been quoted both in Sterling's Reader's Digest article and the NBC broadcast as implicating the Soviet Bloc in the conspiracy.

The Italian Justice Ministry has turned down repeated requests by The Washington Post for information on the conditions in which Agca was held at Ascoli Piceno -- one of Italy's most modern prisons -- on the ground that the secrecy of Martella's investigation needed to be preserved. In particular, ministry officials have refused to say exactly when he was allowed access to the mass media.

The point could be relevant at a forthcoming trial in view of claims that Agca could have gotten at least some of the details about his alleged co-conspirators from the press. In an interview in Sofia, for example, Bekir Celenk maintained that Agca had based accusations against him in May 1982 on a series of articles that had appeared in the Turkish press two months earlier.

Written by one of Turkey's foremost investigative journalists, Ugur Mumcu, the articles in the independent left-wing daily Cumhuriyet described the involvement of Celenk and Ugurlu in the smuggling of goods through Bulgaria. One article, on March 6, noted that Celenk had been staying in Sofia at the same time as Agca and speculated that the two men may have met.

Mumcu based his conclusion on information from a colleague that Celenk had been staying in Sofia's luxury Hotel Vitosha in July 1980. In testimony to Italian magistrates, Agca also said that he had stayed at the Vitosha in addition to a string of other first-class hotels in Sofia. Agca's statement about staying in the Vitosha has never been confirmed independently.

"I have been accused on the basis of the writings of a journalist," Celenk said.

Mumcu, who later gave evidence to Martella as an expert witness, said in an interview that it was conceivable that Agca could have read his March 6 article in Cumhuriyet about Celenk. He recalled that, during a visit to Ascoli Piceno in February 1983 during which he met the pope's assailant for half an hour, Agca complimented him on the "truth" of his articles and seemed to be particularly well-informed about what was happening outside prison.

"I was surprised and asked him how he knew my writings. He told me that his brother Adnan sent them to him from time to time," Mumcu said.

Articles in the Italian, French and Turkish press published in 1982 report that Agca had access to a large number of different newspapers and magazines in several different languages. His private cell at Ascoli Piceno, on Italy's eastern Adriatic coast, also was equipped with a television set.

It is worth pausing here to examine the movements of the principal suspects in the case in the period between May 1982, when Agca implicated the "godfathers" of the Turkish mafia whom he allegedly had met in Sofia, and his identification of three Bulgarian officials as his alleged accomplices in November.

Abuzer Ugurlu, named by Agca as the man who helped him escape to Bulgaria, was in prison in Turkey. He had surrendered to the Turkish martial-law authorities in March 1981 -- two months before the attempted assassination of the pope -- and was under investigation on a string of smuggling charges. Celenk Says He Shrugged Off Agca's Charges

Celenk, meanwhile, was in West Germany between May and September, according to his passport. He now claims that he was well aware of Agca's allegations against him -- but did not take them seriously. One source of information was Milliyet, the Turkish newspaper, which reported on its front page on July 11 that both Celenk and Ugurlu had been directly implicated in the case by Agca in Rome.

The Milliyet article, Celenk said in an interview, coincided with a "chance meeting" that he had had with Omer Mersan in Munich. Mersan who had just returned from an interrogation session in Rome, reportedly told Celenk that Judge Martella was looking for him as well. (This detail has been confirmed by Mersan in testimony to Italian investigators.)

Referring to stamps in his passport, Celenk said he spent October in Yugoslavia and Austria -- and traveled to Bulgaria on Oct. 24, two days before a formal warrant for his arrest was issued by Martella. He has since been placed under surveillance by the Bulgarian authorities, who have refused him permission to leave the country.

The three Bulgarian officials who were to be named by Agca as his "controls" had all stayed on in Rome for many months after the failure of the assassination attempt in May 1981. The first to depart was the assistant military attache at the embassy, Maj. (now Lt. Col.) Zhelio Vasilev, who left Italy on Aug. 27, 1982 -- which, according to him, marked the normal end of his tour of duty.

The embassy's administrative officer, Todor S. Aivazov, left Italy on Nov. 5 on what he has depicted as a routine business trip to Sofia. His return flight back to Rome, he told Italian investigators, was booked for Nov. 26 -- but he canceled it at the last moment after hearing that he could be arrested.

Sergei I. Antonov, the deputy station manager of the Bulgarian state airline, Balkanair, also had remained on in Rome despite the fact that his own office had been searched by police the previous April. Arrested in November 1982, he has been in detention in Italy for nearly two years.

All three Bulgarians were later to insist that the fact that they had stayed on in Italy despite ample opportunities to flee supported their argument that they were not involved with Agca.

"To have remained in Rome for a year and two months after the assassination attempt and the arrest of the would-be assassin is clear evidence that I had nothing to do with the plot. I would have been a fool to remain there all that time had I had been responsible for organizing it," Lt. Col. Vasilev said in a recent interview in Sofia.

By September 1982, the Bulgarian authorities were displaying clear signs of alarm at the direction the interlocking Italian investigations appeared to be taking and the talk of a "Bulgarian connection" to the papal plot. In a statement issued on Sept. 8, the Bulgarian news agency said Agca's testimony "changes so often and is so controversial that we would not be surprised if one fine day, on someone's suggestion and for some promise, he even 'confesses' that the Bulgarians ordered him to kill the pope."

In a move interpreted by many Italian commentators as a kind of insurance policy against such a development, the Bulgarian police arrested two Italian tourists on flimsy spying charges. After being sentenced to long prison terms, the Italians eventually were released unconditionally in 1984. Martella Laboriously Checks Out the Clues

Judge Martella spent the summer of 1982 laboriously following up all the clues contained in the confessions of the pope's would-be assassin the previous May. His opinion of Agca's credibility as a witness appears to have been strengthened when many of the details in his testimony checked out.

One of the new leads enabled the magistrates to track down a second-string "Gray Wolf" living in Switzerland, Omer Bagci, who had helped Agca during his wanderings around Western Europe. Picked up by Swiss police, Bagci, 27, confessed to hiding the 9-mm Browning used in the shooting of the pope and delivering it to Agca at the Milan railway station.

Agca's evidence also helped the West German police close in on Musa Serdar Celebi, 28, the leader of the Frankfurt-based federation of Turkish rightists. Celebi was arrested in Frankfurt on Nov. 3 and eventually acknowledged that he had known Agca as "Murat." He was extradited to Italy the following January.

In the fall of 1982, after a gap of almost six months, Agca started talking to Martella once again. This time his allegations were even more sensational than they had been in May. The attempt on the pope's life, he told the Italian magistrate, had been carried out under the direct supervision of three Bulgarian agents in Rome.

Exactly how and when Agca first came to identify the three agents from photographs is not clear. According to former defense minister Lagorio in his subsequent report to parliament, the "decisive turning point" came in September 1982, when Agca was shown the photo album that had been prepared by the secret services and identified his three Bulgarian "accomplices."

The report by Italian prosecutor Antonio Albano, however, says that the "photographic identity parade" was conducted weeks later: on Nov. 8.

"It was very impressive. He pointed out the three Bulgarians without hesitation," Albano said in an interview later.

The contradiction in timing between the versions provided by the defense minister and the public prosecutor has not been explained. Adding to the confusion is the fact that court records show that on Oct. 28, 1982, Agca claimed that a man photographed running away from St. Peter's Square after the assassination attempt was Aivazov, the administrative officer of the Bulgarian Embassy in Rome. (Seven weeks later this claim was shown to be false.)

According to the Italian Foreign Ministry, it was not until Nov. 11 that Martella inquired about the legal and diplomatic status of Aivazov, Vasilev and Antonov as Bulgarian officials resident in Rome. Arrest warrants against the three Bulgarians were issued on Nov. 24.

At 9:30 a.m. on Nov. 25, Italian antiterrorist police entered a nondescript five-story apartment building on a narrow street in Rome's Nomentana district. After a few moments they emerged with Antonov, a timid-looking, bespectacled man of 34, in handcuffs. The arrest warrant accused him of having taken part directly in the attempted murder of Pope John Paul II in May 1981.

The "Bulgarian connection" had become official.

NEXT: Agca changes his testimony