Was Turkish gunman Mehmet Ali Agca acting on his own when he shot Pope John Paul II in St. Peter's Square--or was he part of an international conspiracy? The Russians, even during the Soviet era, wouldn't have been so crazy as to try something like that. The Bulgarians weren't smart enough. The Turks, Agca's nationality, had no reason to engage in such a plot. If the CIA knows who was behind the assassination attempt, it's not saying--although the agency does maintain that it wasn't the Russians. The Italian police searched around vainly for co-conspirators, then officially closed their investigation. Meanwhile, many--apparently including some Vatican officials--continue to believe that powerful people who wanted John Paul dead were behind Agca when he pulled out his gun on May 13, 1981.

Now, journalist Tad Szulc, a longtime reporter for The New York Times and author of the best biography to date of the present pontiff, "Pope John Paul II: A Biography" (Pocket Books) , claims to have unraveled the mystery--and he concludes that the conspiracy theorists are correct. His conclusions, based on documents he found and high-level officials he interviewed while researching the biography, appear in a brand-new novel, "To Kill the Pope: An Ecclesiastical Thriller" (Scribner's). Yes, Szulc's latest book is fiction, but according to its author, it is based strictly on the facts he uncovered. He chose the novel format only because his sources forbade him to disclose real names and specific details. Behind the attempted murder, Szulc maintains, was an international consortium of religious fundamentalists probably directed by right-wing French Catholics who resemble the followers of the late excommunicated Archbishop Marcel Lefebvre. In the eyes of some of these hard-rightCatholics, Szulc theorizes, John Paul II, although conventionally regarded as a conservative, was a dangerous radical who had no intention of reversing the Second Vatican Council.

The release of "To Kill the Pope" coincides with a new wave of interest in the papal assassination attempt, occasioned by the Vatican's revelation after many decades of the so-called "Third Secret of Fatima." The Catholic Church is interpreting the prophecy, reportedly made by the Virgin Mary to three Portuguese peasant children, as a foretelling of the assassination attempt on John Paul II, which coincidentally occurred on the 64th anniversary of the first appearance of Mary at Fatima, Portugal, on May 13, 1917. According to the Vatican, Mary described a "bishop clothed in white" (the color of John Paul's papal cassock) who "falls to the ground, apparently dead, under a burst of gunfire."

The unveiling of the Third Secret, together with John Paul's recent pilgrimage to Fatima to commemorate the 83rd anniversary of the 1917 visions and give thanks for his survival in 1981, has pushed the assassination attempt--and the mystery surrounding its motivation--back onto center stage. Agca, currently serving a life sentence in an Italian prison for the shooting, gave a talk-show interview on May 15, in which he hinted that unnamed authorities were pressuring him to come up with some co-conspirators in order to win an early release. He continues to insist, however, that he acted alone.

This is exactly the thesis that Szulc rejects--and that he insists in his book the Vatican has rejected as well. Szulc contends that high-placed church officials were dissatisfied with the absence of an explanation for the shooting--especially its motivation--and ordered their own investigation. According to Szulc, the investigator they chose was an American Jesuit priest. Who else?

"To Kill the Pope" is a novel, but according to its author, it is an only lightly fictionalized version of the truth, changing John Paul's name to Gregory XVII and that of his would-be assassin to Agca Circlic, a Turk who belongs to a terrorist group. Szulc is a respected journalist, which lends credibility to his conclusions. Moreover, his story has the ring of truth about it. I cannot endorse his account as certainly true, but I can say that it is by no means improbable.

Szulc maintains that a fundamentalist Turkish group chose Circlic to be the killer at the request of their "fraternal brothers" in a Muslim fundamentalist group in southern France. They in turn were apparently acting at the request of a Catholic fundamentalist group, not unlike the followers of Lefebvre, who was excommunicated and died later in the decade.

Such an argument will seem highly improbable to those who don't understand the religious culture of the south of France and the self-righteous intensity of the Catholic "Integrists" who are concentrated in the region. After Charles Martel routed Arab invaders at Tours in 732, the Arabs fell back to the Pyrenees. From their mountain redoubts, they regularly attacked France, and the French (particularly under Charlemagne) regularly counterattacked and began to eat away at the Arab holdings in Spain. It was the beginning of the "Reconquista," the 700-year struggle to recapture Iberia from Arab control. The south of France was for centuries essentially a militarized border region.

Moreover, as the years went on, Muslim influence from Spain and eastern religious movements filtering into France through the even-then polyglot port of Marseilles, made southern France, particularly the region called Provence or Languedoc, a hotbed of confusing and often fanatical religious movements.