A laborious three-year investigation by Italian authorities into the attempted assassination of Pope John Paul II in May 1981 has removed any lingering doubt about the existence of a conspiracy to kill the pontiff.

The key issue now is not whether Turkish gunman Mehmet Ali Agca acted alone, as he claimed initially, when he shot the pope in St. Peter's Square -- but who were his accomplices and what was their motivation. The answer is more complex than might appear from the formal indictment announced yesterday of three Bulgarians and four Turks as Agca's co-conspirators.

The implications of a possible Soviet Bloc connection to the attempted assassination of a pope for East-West relations are immense, and public attention has focused on the accusations against the three former Bulgarian officials in Rome. Relatively little attention has been paid to the parallel charges that have been made against members of an extremist right-wing Turkish terrorist group known as the Gray Wolves.

The distinction between the two groups of defendants is important because it raises the possibility of two entirely different types of plots against the spiritual leader of 800 million Roman Catholics. It will be up to the court to decide, on the basis of the evidence submitted, which scenario is more plausible.

The prosecution case, as outlined in a report by the public prosecutor earlier this year, is that the assassination attempt against the pope was organized on the orders of a senior Soviet Bloc politician. The motive was the suppression of social and political upheavals in Poland through the removal of the religious inspiration provided by the election of the first Polish pope in history.

The prosecutor will attempt to demonstrate that Agca was recruited by agents of the Bulgarian secret service. Logistical support, according to this scenario, was provided by a network of emigre Gray Wolves in Western Europe as well as by the indicted Bulgarians in Rome.

The alternative scenario is that the plot to kill the pope involved only Turks. Defense counsel for the accused Bulgarians will attempt to persuade the court that there is no convincing evidence linking their clients to either Agca or any of the other four Turkish defendants.

According to this argument, the key actors in the conspiracy were a group of Gray Wolves from eastern Turkey who already had been responsible for the murder of a prominent newspaper editor in Istanbul and fled to Western Europe following the September 1980 military coup. The motivation to kill the pope, this scenario argues, came from a mixture of antiwestern Islamic fundamentalism and a desire to promote anarchy in Europe.

At a news conference after filing his report, Italian examining magistrate Ilario Martella insisted that there was convincing evidence of a plot to kill the pope. But he insisted several times on the need for caution in drawing political conclusions about the nature of the conspiracy before all the evidence had been heard in open court.

The full 1,243-page text of official indictment has not been made public yet. Martella, whose conduct of the probe has been characterized by prudence and an aversion to publicity, has not gone as far in his public comments as prosecutor Antonio Albano in accusing Communist governments of being behind the assassination attempt.

In his report, which was filed in court earlier this year, Albano said that a "politician of great power" had ordered the killing of the pope "in accordance of the higher needs of the Soviet Bloc" following the eruption of the Polish crisis in 1980-81.

Confirmation that there was some kind of plot to kill the pope is provided by the proven presence of another gunman in St. Peter's Square on the day of the assassination attempt. Martella told the news conference that eyewitness testimony plus an examination of film clips of the shooting demonstrated that a third shot was fired at the pope by someone standing next to Agca.

The second gunman, according to Martella, was Oral Celik, one of Agca's closest friends and a influential member of the Malatya faction of Gray Wolves. Evidence pointing to Celik's presence in the square includes a photograph taken by an American tourist as well as Agca's own testimony, Martella said.

Agca claimed that two of the Bulgarian suspects were also waiting in the square to assist in an escape plan that would have smuggled him out of Italy in a diplomatic vehicle or truck. A truck left the Bulgarian Embassy in Rome shortly after the assassination attempt.

At the news conference yesterday, Martella said that the evidence for a relationship between Agca and the Bulgarian accused is circumstantial -- despite the contention of the pope's would-be assassin that he held a series of meetings with the three men in Rome bars and restaurants as well as their private apartments. The Bulgarians claim that they never met Agca.

Bulgaria lodged a formal protest with Italian authorities Saturday over the indictment of its three citizens, United Press International reported.

[The Bulgarian state news agency BTA said Giovanni Battistini, Italy's ambassador in Sofia, was summoned to the Foreign Ministry and handed a "sharp protest." Bulgarian radio said the ambassador was told the indictment was a "prearranged provocation against Bulgaria and against real socialism."]

In testimony to Italian magistrates, Agca was able to cite details about the movements, habits and physical appearance of his alleged Bulgarian accomplices that later proved to be accurate.

Citing one such example, Martella said that the pope's would-be assassin had claimed to have met Bulgarian Embassy administrative officer Todor S. Aivazov in Sofia in July 1980. When the investigators went to check, they discovered that Aivazov indeed had been in Sofia during that period.

By contrast to the circumstantial evidence of the involvement of the Bulgarian defendants, the investigators have concrete proof of a relationship between Agca and the accused Turkish rightists.

Two of the Turks have acknowledged that they had dealings with Agca -- even though they insist that they knew him under another name and were not involved in the plot to kill the pope. A third, Celik, the alleged second gunman, can be shown to have been associated with Agca over a long period of time.

The lack of hard evidence against the Bulgarians has led many magistrates and lawyers with knowledge of the case to conclude that it will be very difficult to secure convictions against them in the forthcoming trial.

The defense will seek to show at the trial that Agca could have learned the details about the accused Bulgarians either before or after his arrest and subsequent conviction for shooting the pope. It was not until the fall of 1982 -- 18 months after the attack -- that Agca identified the Bulgarians as his accomplices from a photo album shown him by the magistrates.

The prosecutor has accepted as "surprising but in fact probable" Agca's own contention that some of his testimony against his alleged accomplices was based on newspaper and television reports to which he had access in prison. It is not possible, however, that he could have learned all the details of the accused Bulgarians from the mass media.

For the defense, this fact implies that Agca must have been fed information about the accused Bulgarians. For the prosecution, it is circumstantial evidence of a relationship between the Bulgarians and the man who shot the pope.

Prosecutor Albano has argued that, if it can be shown that Agca knew the Bulgarians personally, it is reasonable to make the logical deduction that the accused Bulgarians must have been involved in the plot to kill the pontiff.

The trial is likely to be complicated further by the contradictions and retractions in Agca's own testimony. Martella told journalists that he had devoted four out of 11 chapters in the major part of his indictment to examining the question of Agca's personality and his behavior under interrogation.

Martella has ordered the dropping of charges against a Turkish businessman who figured prominently in Agca's early testimony as a key accomplice. The businessman, Omer Mersan, was accused of perjury but was absolved after he changed his testimony on a relatively minor point.

The judge also ordered the shelving of all charges arising from Agca's allegations of a plot to kill Solidarity leader Lech Walesa. The judicial investigation into the affair has now been formally closed.