It was described as "the trial of the century," Exhibit A in the case against the Kremlin for state-sponsored terrorism. If it could be shown that the Soviet Bloc was involved in the attempted assassination of a pope, the future of East-West relations could be in jeopardy.

In fact, the 10-month trial of three Bulgarians and five Turks on charges of plotting to murder Pope John Paul II failed spectacularly to live up to its advance billing. Attempts to prove a "Bulgarian connection" to the papal plot fell apart because of lack of evidence, a poorly argued prosecution case and the bizarre behavior of the state's star witness, Mehmet Ali Agca.

Agca, a Turkish gunman who shot and seriously wounded the Polish-born pontiff in St. Peter's Square on May 13, 1981, provided the bulk of the testimony against the Bulgarian defendants whom he depicted as his direct accomplices in the assassination attempt. In pretrial hearings, he described a series of meetings with the Bulgarians in Rome and alleged that he had received the equivalent of $1.2 million to assassinate the pope.

Despite hearings in half a dozen countries and the calling of more than 50 witnesses, the court was unable to trace any of the money allegedly paid to Agca by the Bulgarian secret services. It was unable to shake the blanket denials of the Bulgarian defendants that they had ever known Agca. Indeed, it failed to produce independent confirmation for any direct Soviet Bloc role in the assassination attempt.

At times, the proceedings degenerated into farce as Agca, 28, sought to drag everybody into the plot, from the KGB to the CIA to the Italian secret services. Predicting the end of the world, Agca presented himself to the court at various times as the reincarnation of Christ, an international terrorist to compare with "Carlos" and an expert in human behavior "greater than Charles Darwin and Sigmund Freud put together."

By the end of the trial, presiding Judge Severino Santiapichi admitted to reporters that he was physically and mentally exhausted, having failed to get anywhere near the truth. The only clear victor from the proceedings seemed to be the pope's would-be assassin. Agca was able to satisfy his obvious craving for publicity in addition to keeping the real reasons for the assassination attempt shrouded in mystery.

The trial is over, but a number of questions remain about a case that was seized upon by conservatives in the United States as evidence of Soviet involvement in international terrorism and of western complicity in "covering up" the crime. Among the most important: the relationship between Bulgaria and a Turkish smuggling ring and flaws in the Italian judicial investigation into the papal plot.

*Bulgaria and the "Turkish mafia." The trial failed to resolve the mystery surrounding Agca's stay in the Bulgarian capital, Sofia, in July and August 1980, at a time when he was wanted in neighboring Turkey for a sensational political murder.

Evidence produced at the trial showed that Agca traveled to Bulgaria on a false Indian passport after escaping from prison in Turkey. In Sofia, he had contacts with members of a powerful crime syndicate, the "Turkish mafia," that had been involved in smuggling arms and other contraband into Turkey with the tacit approval of Bulgaria's Communist authorities.

Bulgaria has failed to provide a convincing explanation for how it was possible for one of Turkey's most wanted murderers to stay in the best hotels in Sofia for at least six weeks without attracting attention.

According to Ilario Martella, the Italian magistrate who handled the preliminary investigations, the Bulgarian authorities falsified documents relating to Agca's stay in Sofia. Martella argued that the changes were designed to make it appear as though Agca must have been lying about the dates of alleged meetings with Bulgarian officials and a leading member of the Turkish mafia, Bekir Celenk, one of the defendants.

The nature of the chain of relationships among Agca, a right-wing Turkish terrorist group known as the Gray Wolves, the Turkish mafia and the Bulgarian authorities remains mysterious. Proponents of a "Bulgarian connection" to the papal plot have argued that the Communist authorities used the mafia to hire Agca to shoot the pope. A rival theory holds that the Gray Wolves exploited their contacts with the mafia to hide Agca in Sofia after his escape from a Turkish prison.

*The Italian investigation. The trial demonstrated glaring holes in the pretrial investigation into the papal assassination attempt. Martella concentrated on checking out Agca's "confessions" in prison and his allegations against the Bulgarians, while largely overlooking proven links between Agca and right-wing Turks. A new investigation has been opened by the Italian authorities to plug the gaps in the initial, two-year inquiry.

Although little evidence has emerged to support the defense contention that Agca was persuaded to implicate the Bulgarians by the Italian secret services, the way in which the pope's would-be assassin gave his testimony raises disturbing questions. A study of Martella's 1,200-page indictment shows that the Turk could have learned some of the incriminating details about the Bulgarian defendants in the course of the investigation.

Procedural lapses in the investigation included the fact that Agca was allowed access to a Rome telephone directory that contained numbers he later gave to Martella to support his contention that he had a personal relationship with the Bulgarians.

In giving testimony, Agca repeatedly changed his description of his alleged Bulgarian accomplices. Some of these changes occurred even after Agca had been shown a photo album that contained the pictures of 56 suspects, all of them Bulgarians.

The lawyers for the Bulgarians have complained that, unlike normal identity lineups in which suspects and nonsuspects appear before a witness, this particular album only contained pictures of Bulgarian citizens in Rome. It was first shown to Agca in November 1982, about six months after it was compiled by the Italian secret service for use in another criminal case. Although there is no evidence that Agca had seen the pictures before, the existence of the album was known in Italian judicial circles.

Perhaps the most serious error committed by the Italian investigators, however, was their apparent willingness to believe uncorroborated statements by Agca. Documents in possession of the court show that the Turkish gunman lied and changed his story hundreds of times while under interrogation both in Italy and Turkey. Yet the pretrial magistrates continued to give him the benefit of the doubt.

Influenced by Italy's Roman Catholic traditions, Italian magistrates have tended to give more weight than their Anglo-Saxon counterparts to the confessions of "repentant" criminals, or pentiti as they are known here. In open court, however, Agca's professions of repentance looked more like an attempt to ridicule Italian justice. His lack of credibility as a witness delivered a fatal blow to the case against both the Bulgarians and the Turkish defendants.