The unsuccessful plot to kill the Pope is about to swirl back into the limelight. Next month, the findings of Italian Judge Ilario Martella's meticulous investigation of the case, concluded in mid-December after nearly 21/2 years, are scheduled to be made public.
In addition, the Italians are wrestling with the decision as to whether to put Sergei Antonov, former head of the Bulgarian airline's office in Rome, on trial. Mr. Antonov has just been returned to prison after spending a month under house arrest. He has been investigated as an accomplice in the plot.
Musa Serdar Celebi and Omar Bagci, two Turks who were extradited to Italy early last year, may also be tried as accessories.
The Bulgarians and the Soviets, whose secret services frequently work closely together, have heatedly denied any involvement in the 1981 attempt by a young Turk, Ali Agca, to assassinate Pope John Paul II - even though Agca declared publicly last July that he knew he was working for the KGB, the Soviet secret police. Judge Martella, too, is reported to have built an unusually strong case for Bulgarian conspiracy with Soviet backing.
Instead the two Communist governments have de-rided such reports. The Soviets have put out a stream of reports alleging that the United States was behind the plot. And the Bulgarians have pushed a campaign to create the impression that the case against Antonov had collapsed because Agca had been proved a liar.
No observer in Rome will rule out the possibility of a deal whereby Antonov, though proven guilty in Italian law, might still be traded for the two young Italians arrested in Bulgaria in December 1982 and convicted of espionage on apparently trumped-up charges.
Current theories being propagated about the origins of the plot to kill the Pope can be summed up under four headings:
1. The Russians did it, working through the Bulgarians.
2. Somebody else did it.
3. Agca was mad.
4. The Americans did it.
All serious judicial and professional investigation that has been done since 1981 strengthens the first hypothesis. So does careful examination of the other three.
We know that Agca was recruited young and given unusual handling, including training by the Palestine Liberation Organization. Large sums of money were deposited in banks for him before he committed his first spectacular murder of an editor in Turkey.
Hidden sponsors then extricated Agca from prison and got him out of Turkey into Iran. Several mysterious months later he turned up in Bulgaria, where his links to Turkish smugglers (such as Bekir Celenk and Abuzer Ugurlu) and the Bulgarian secret police are documented. Then he was set on a circuitous but well-financed path to Rome.
Who masterminded all this?
A non-Russian theory requires plausible explanations. Could it have been Islamic fundamentalists? Turkish arms and narcotics traffickers? The PLO? The Italian Mafia? Anti-Polish elements in the Catholic hierachy?
The problem of motivation is crucial. None of the above groups can be shown to have a sufficiently compelling motive to develop and sustain, over a long period of time, such an elaborate and costly scheme entailing classic clandestine intelligence techniques.
Agca was only a nominal Muslim. He had no fundamentalist ties. As a student he was oddly apolitical. There is no evidence that he was ever even a ''Gray Wolf'' (a rightist Turkish nationalist). He was never involved in smuggling.
Perhaps then he was simply mad - like John Hinckley, who shot President Reagan?
Repeated psychological tests have shown Agca to be not only sane, but also above-average in intelligence. Like the ''somebody else'' theory, the ''madness'' theory falls apart in the face of fact.
Where does this leave us?
With the Americans, of course, say the Kremlin's propagandists. The Russians started to push the blame onto the United States immediately after the shooting - insinuating that William Wilson, President Reagan's personal representative at the Holy See and now ambassador designate, was angered by the Pope's attitude on Jerusalem and El Salvador.
Disinformation initially spread in 1981 has had to be bolstered by taller and taller tales. The result is an incredible hodgepodge of alleged CIA conspiracy, Brzezinski and Kissinger actions, and orders from the Reagan White House.
Moscow's prestigious Literaturnaya Gazeta pulled all this together in a series of articles last June and July. The series has been translated into many languages and is being energetically distributed as a booklet entitled ''On the Wolf's Track.'' (The Bulgarian Embassy in Washington has mailed out thousands of copies and will readily supply one to any caller.)
If the Russians and Bulgarians could come up with a plausible alternate explanation of the plot, it would be sure to get some acceptance. There are many individuals - and governments - in the world who would rather not acknowledge that Kremlin leaders actually planned such a deed. But it is hard to find anyone ready to buy the Soviet thesis that the US did it.
Washington, like other Western capitals, continues to maintain discrete silence about the plot. Thus some journalists have concluded that no one in the Reagan administration any longer believes in the Bulgarian connection. Others contend that President Reagan and CIA Director William Casey are engaged in a deliberate cover-up of evidence of Soviet complicity. Both contentions seem exaggerated.
A high administration official who had canvassed views among CIA and State Department Soviet specialists told this writer the other day, ''I cannot find anyone there who doesn't believe the Kremlin was behind it!''
Oleg Bitov, Literaturnaya Gazeta's chief foreign editor, defected in Italy in September and is now being interrogated in England.
Soon after he disappeared from the Venice Film Festival, Moscow claimed he had been kidnapped as part of the US cover-up of the plot to kill the Pope. But Bitov may soon tell us more about Moscow's efforts to put the blame on the US.