The Italian judges investigating the plot to kill Pope John Paul II and other apparently related cases seem determined to probe the ''Bulgarian connection'' to its roots.
Judge Ilario Martella and his colleagues have collected far more information than has been released publicly. But what is already known provides strong support for allegations of widespread Bulgarian subversion - and, hence, of Soviet involvement through Moscow's close ties with the Bulgarian secret service.
And the investigations still are growing in scope almost daily. What started as a probe into Turkish terrorist Mehmet Ali Agca's 1981 attempt to assassinate the Pope has widened into investigations of international drug and arms smuggling, of a purported attempt against the life of Polish labor leader Lech Walesa, and of links with the kidnapping by Red Brigade terrorists of American Gen. James Dozier.
Several hundred people have been arrested in connection with these probes. The tentacles reach out across Europe and apparently into the Middle East.
Some of the main themes of these investigations can be summed up as follows:
* The core of the allegations that Bulgaria was behind the plot against the Pope rests on the thesis that Agca, who was convicted on July 22, 1982, for shooting the Pope, had ties to the Bulgarian secret service. Tending to back up this thesis has been the questioning of Sergei I. Antonov, the Rome bureau chief for Bulgarian Airlines, who was arrested Nov. 25 last year for complicity in the papal assassination attempt.
Antonov's appeal was rejected at the end of January. His alibi that he was not in St. Peter's Square at the time of the May 13, 1981, shooting has been set back. An Italian woman who earlier said she recalled having talked to him then in his office now says she remembers that it was three hours after the attempted assassination that she telephoned Antonov about an air reservation.
Antonov is also under investigation for possible complicity in the alleged plot against Mr. Walesa. He and Agca were warned last month by another judge, Ferdinando Imposimato, that they could be charged with plotting to kill Walesa when he visited Rome in January 1981. At that time, Walesa and his Solidarity union were directly challenging the Polish Communist authorities - with support from the Roman Catholic Church, including the ''Polish Pope.''
In addition to Antonov, three other Bulgarians have been warned they are under investigation concerning the Walesa plot. They are Vassiliev Kolev, Teodoro Ayvazov, and Ivan Dontchev, all former employees at the Bulgarian Embassy in Rome. Ayvozov, like Antonov, has also been accused of complicity in a plot to kill the Pope.
The Bulgarian government has denied that Antonov was connected with either the attack on the Pope or the alleged plot against the Polish union leader.
* Another of those warned by Judge Imposimato about possible involvement in the Walesa case is Luigi Scricciolo. The Italian labor leader was arrested in February 1982 on suspicion of working as an agent for East bloc countries and trying to establish links between Italy's Red Brigade terrorists and Bulgarian secret agents. He is charged with helping the Red Brigades to kidnap US General Dozier from his home in December 1981. Dozier was rescued six weeks later by the Italian security services.
Scricciolo, who admitted informing on Walesa, has confessed to working for Bulgarian intelligence since 1976. He also helped to organize Walesa's trip to Rome in January 1981.
Scricciolo has identified several alleged Bulgarian agents with whom he said he had dealings. Some of them left Italy three or four years ago. Security officials say they have traced some of these Bulgarians to a multi-billion-dollar Bulgarian-backed drug and arms smuggling ring.
* Nearly 300 people have been arrested by order of Judge Carlo Palermo, who has been looking into alleged Bulgarian ties to several Middle East countries, the Palestine Liberation Organization (PLO), and leftist and rightist terrorists in Turkey. Palermo's investigations have led him to believe that the northern Italy-based network, which has been shipping arms to the Mideast in exchange for drugs, may have connections with the Bulgarian capital of Sofia.
Particular attention has been focused on Bekir Celenk, a Turkish businessman suspected of participating in the ring. Both Judges Martella and Palermo have issued arrest warrants for Celenk. Celenk allegedly met with Agca in Sofia in the summer of 1980 and seems to have been a key element in activating the final stage of the plot against the Pope. Celenk is currently under Bulgarian control, meaning he is free but not allowed to leave the country.
This past week Judge Palermo went to Sofia to interrogate Celenk. The discussion went on for several hours and Palermo commented afterward that it had been very important and useful.
* The testimony of Musa Serdar Celebi, a rightist Turkish leader extradited from West Germany who now admits to knowing Agca, may shed some light on the papal attack. Suspected of being a key link between Agca and Turkish Mafia leaders in Bulgaria, Celebi is alleged to have offered over $1 million for the shooting of the Pope.
* A recent development involves Horst Grillmayer, the Austrian arms dealer to whom Agca's gun has been traced, and who had been missing since June 1981. On Jan. 11, 1983, Grillmayer was caught by Austrian customs officers coming out of Czechoslovakia with a falsely documented truckload of 345 pistols and seven Soviet-made sniper rifles. Nine persons have been arrested in the Austrian investigation of Grillmayer, and Italy has requested his extradition.
Why Grillmayer, who had been forgotten, was permitted to reemerge into the West is as much a mystery as why Antonov was not recalled to Bulgaria long ago.
Perhaps these failures betray overconfidence bred by years of Bulgarian and Czechoslovak subversion in Italy and elsewhere, when nobody was caught and nobody blew the whistle. That era has clearly come to an end. Next: Turkey reexamines Agca case