Important new information about the unsuccessful plot to kill the Pope is expected from a reopened murder investigation in Turkey. This, in turn, may help to answer the ultimate question in the papal case: whether or not the Bulgarians, and hence the Soviets, were involved in the shooting of Pope John Paul II by a Turkish gunman last year.
Some American ex-government officials have recently expressed their conviction that the abortive plot against the Pope had Kremlin support:
* ''It had to be the Soviets,'' former Secretary of State Henry Kissinger said in a recent television interview. ''The Bulgarians have no interest in coming after the Pope.''
* ''Those who know the reality of Eastern Europe automatically deduce that the Soviet Union was in command of the operation,'' former National Security Adviser Zbigniew Brzezinski told the Turin newspaper La Stampa this past weekend. ''Only the KGB could have been its instrument and (Soviet leader Yuri) Andropov dominated it for 15 years. The logic in this affair is irrefutable.''
Last week the Turkish government decided to reopen a case involving the Pope's would-be assassin, Mehmet Ali Agca. He was sentenced to death in absentia for the 1979 shooting of Abdi Ipekci, editor of the liberal Turkish newspaper Milliyet.
''Reexamination of this murder - which has never been satisfactorily explained - is bound to shed new light on the whole background of terrorism in Turkey,'' observes a US diplomat who was assigned to Istanbul at that time.
The notion that it was the work of rightist nationalists has been increasingly questioned in Turkey. Former National Action party leader Alparslan Turkes was charged with instigating it when he was put on trial nearly 1 1/2 years ago. The trial produced no evidence to substantiate the charge.
''I find it almost impossible to believe that Turkes would desire or sanction the murder of Ipekci,'' said a Turkish official recently who knew Turkes when he was a member of coalition governments during the 1970s. ''But I am not at all sure he knew what was going on in his party. It appears increasingly likely that the party was infiltrated by foreign agents bent on destroying Turkish independence. Rightist nationalism was only a cover for them.''
This interpretation has been given substance by reports of the interrogation in Frankfurt of a new figure in the papal plot, Musa Serdar Celebi. Celebi, a fugitive from Turkey who had been accepted as a rightist leader in the Turkish community in West Germany, was arrested in Frankfurt several weeks ago at the request of Italian authorities. He is said to have admitted meeting with Agca in Milan in December 1980 and in Zurich in March 1981. According to the Turkish press, Agca has confirmed the meetings to Judge Ilario Martella.
At the March meeting in Zurich, Celebi is said to have admitted to proposing to Agca that he assassinate the Pope. And in offering Agca 3 million marks ($1.3 million) to do the job, Celebi is also said to have admitted as acting on behalf of the notorious Turkish smuggler Bekir Celenk. Celenk, who lives in Bulgaria and allegedly met Agca in Sophia's Hotel Vitosha in summer 1980, returned hurriedly to Bulgaria from London in early December when Judge Martella issued an arrest warrant for him.
Mr. Celenk's claim to being ''simply an international businessman'' is widely regarded both in Italy and Turkey as being as specious as the Bulgarian government's announcement that they have arrested and are interrogating him.
According to a Turkish press interview with a West German prosecutor, Celebi says Agca gave final agreement to hisassassination offer in a telephone call from Majorca in April 1981. German and Italian officials are assumed to know more than has already been revealed.
Nevertheless, Celebi's reported admissions begin to fill out a plausible picture of how the final phase of the plot against the Pope may have been activated. Coupled with new information from Poland, they may also provide insight into why the attempt on the Pope bears marks of improvisation.
According to an authoritative Polish source, World War II resistance hero and current historian and author Jan Nowak, Stefan Cardinal Wyszynski, symbol of Polish national resistance to communism for three decades, was diagnosed as suffering from an incurable disease in mid-March 1981 and given no more than six weeks to live.
''The seriousness of his condition was confined to a very small circle until late April,'' Mr. Nowack states. ''But the Soviets must have had agents in Warsaw who reported it to them. They could only have welcomed his demise. But why not seize the occasion to demoralize Poland decisively by eliminating the bothersome Polish Pope at the same time?''
According to this theory, which is now being weighed in Washington, the KGB would have rushed to put into effect a standby plan to do away with John Paul II in hope of bringing the accelerating push for freedom in Poland to a brutal halt. The Bulgarians would have been given the signal to activate Agca rapidly, and some improvisation - reflected in the carelessness many professional intelligence operatives have noted in the Bulgarian modus operandi - was inevitable.
As Washington goes back to work after the Christmas holidays, doubts about the Eastern origin of the plot against the Pope has given way to expectation that more substantial evidence of Bulgarian involvement and Russian backing is likely to come to light in the weeks ahead as investigations in Italy, Germany, and Turkey proceed.
Press reports of skepticism about the plot in West German and Israeli intelligence circles are discounted by serious professionals, for it is evident that the Germans have exerted themselves to secure information such as that which Celebi has provided. The Israelis are known to have given the Turkish government quantities of PLO files captured in Lebanon on Turkish terrorists both left and right. These may shed light on Agca's early PLO training, which might have begun as early as 1976 and would have been the first step on the path which led him to St. Peter's Square in Rome.