A week after the attempted assissination of Pope John Paul II in St. Peter's Square, the only thing known for certain about Mehmet Al Agca, the mysterious Turkish citizen being held for the assault, is that he is an cold professional killer with nerves of steel an impenetrable and devious mind.

Five days of intensive interrogation by a team of Italian magistrates have yielded up neither a convincing political motive nor the identity of the organization behind Agca -- if indeed there was any -- involved in the attempt on the pontiff's life.

Nor have Italian authorities even with the help of the best police intelligence of Western Europe, been able to verify his incredible tale of wandering across six to eight European nations, living at times like a rich tourist despite being a wanted criminal. Even his highly probable presence in West Germany has not been confirmed by Bonn.

Because of his high life style and the large amount of money found on him, no one here doubts Agca's travels were being funded by some group, although Agca himself continues to deny any such ties.

There is a strong suspicion he got the money from one or more of the 129 branches in Western Europe of the outlawed Turkish Nationalist Action Party. The rightist group had, and probably still has, 87 affiliates alone in West Germany, where more than a million Turks live.

One Italian Interior Ministry source said, however, "We don't see any motivation for a right-wing group wanting to shoot the pope."

After first leaping to the conclusion that Agca must have pulled the trigger on behalf of some terrorist group acting with a precise design, the Italian police and press have backed off and now seem about evenly divided between belivers in an "international plot" and "lone-man" theories.

An Italian police source was quoted here yesterday as saying there was a "50-50 chance" Agca acted alone.

Such a statement from an Italian police official is in itself something, since Italians, besieged by terrorism of both the left and right for years now, tend with some justification to see plot and purpose behind every act of violence.

What seems to be emerging from Agca's prolonged interrogation, together with reports from Turkish authorities, is deeply disturbing, namely that there are dozens of Agcas on the loose now in Europe. One Turkish diplomat called them "unsable desperadoes," while an Italian Interior Ministry official dubbed them "mercenary terrorists."

One theory expressed by some Italian authorities is that there may even exist a "terrorist center" to provide professional killers to extremist groups.

"There are hundreds of unsable desperadoes like Agca walking around outside Turkey, all under 25 years of age," said Turkish diplomat.

In any case, Agca's act and his seemingly offhand comments about planning to assassinate Queen Elizabeth or U.N. Secretary General Kurt Waldheim here served to project the danger of international terrorism to the forefront of world attention as never before.

"He seems to be saying no one in the world is safe from terrorism now," said one Vatican watcher.

His act has also laid bare how ill-prepared to deal with the danger Western Europe remains after years of mourning terrorist activity. For despite multiple warnings by Turkish authorities to the West German and Italian police, including at one point information regarding in which Milan restaurant Agca was eating, neither government was able to nab him.

The picture that has emerged from police and press accounts of the 23-year-old convicted rightist Turkish terrorist is full contradictions -- as if he harbors two or more personalities within him.

On the one hand, Acga has been portrayed as a professional assassin, askin in mind and action to Frederick Forsythe's famous character in the novel "The Day of the Jackel." In fact, the hardened terrorist aspect of the man seems chiseled into the angular bones of his face and the detached glance of his eyes.

Turkish sources attribute to him possibly as many as four murders -- two in West Germany of other rightist exiles and two in Turkey -- and say they are certain of the latter, including that of the editor of the Turkish newspaper Milliyet, Abid Ipecki, in February 1979, and of an informer after Agca's escape from prison 10 months later.

He has been detailed and precise in his answers to questions of minor importance, deliberately vague to those of major account, while carefully avoiding any damaging contradictions in his original story, that he acted alone.

Yet for all his proven criminal cunning, the attempted assassination of the pope was in some way highly amateurish. There is no evidence, for example, that he carefully cased St. Peter's Square in Jackal-like fashion before attempting to shoot the pope.

Witnesses have testified he asked about the route of the pontiff's open jeep through St. Peter's Square a few hours before he acted. In addition, he had picked Sunday as an alternate date for his attack, when the pontiff stands high above the crowd in the fourth floor of the Apostolic Palace -- a position that would have made it almost impossible for Agca to hit the pope with the 9 mm Browing pistol he used.

Nor had Acga bothered to plan an escape route other than to obtain information about how to get from the square to the railroad station.

"He was not the kind of cold, thorough-planning person that the Jackal was," remarked one Western diplomat who has followed the event closely.

His character and motives seem just as contradictory as his styles of action. Turkish sources familiar with the interrogation of Agca after his murder of Ipecki fascist" reflecting the views of the Nationalist Action Party to whose young wing, the Gray Wolves, he belonged.

Yet, in the prepared statement found in his hotel room after the shooting, Agca spoke of acting to protest the "imperialistic crimes" of the United States and the Soviet Union.

"In the first crime, he was a narrow-minded fascist, but here he is a broad-minded Maoist," said one of the sources.

Since then during his questioning, Agca has issued fulminating statements on behalf of Islam. "I shot at the pontiff because I considered him responsible for the Western reaction against Islam and the leader of the crusade that is being waged against my faith," the Italian News Agency quoted him today as telling his interrogators earlier in the week.

The evidence that Agca is a Moslem fanatic is slim. He has reportedly not asked to be given a copy of the Koran, or shown any other sign of special attachment to his religion.

"Agca is a Moslem but not a fanatical Moslem," remarked an Interior Ministry official familiar with the report of his interrogation.

There is a Moslem theme running through his pronouncements and grisly history as a terrorist. As far back as the 1979 visit of the pope to Turkey, Agca threatened to kill the pontiff in a letter to Milliyet, apparently out of a mixture of ultranationalist and religious fervor.

Also, at the time of the takeover of Mecca by Moslem extremists that same year, Agca blamed the United States and Israel and threatened to kill their consuls-general in Istanbul -- among others.

This has led to the line of thinking among some Italian officials and commentators that Agca was an "obsessed maniac" who simply took advantage of his presence in Italy to come to Rome and take a shot at the pope.

This theory holds that even if the rightist Turkish group supporting him did not have the killing of the pope as one of its prime objectives, it did not really try to stop him either.

Other Italian officials, as well as outsiders, are sticking to various hypotheses that there must have been a group and religious or political purpose behind the attack on the pope's life.

"It could be a Moslem group in opposition to the Catholic Church," remarked a high-ranking Italian official, "or there could be a Polish connection," he added, nothing Agca alleged he was in Bulgaria after his escape from a Turkish prison in November 1979.

The Bulgarians might be upset enough by the alternative to communism evolving in Poland and the strong backing of the Catholic Church, as well as of the Polish pope, to the Solidarity independent union movement to encourage Agca in his endeavor, according to this "hypothesis."

One Western diplomatic source, however, called this theory "off the wall," together with that holding some extremist Moslem group responsible.

After a meeting here March 18 between Palestine Liberation Organization official Farouk Kaddui and Cardinal Agostino Casaroli, the Vatican's secretary of state, relations have never been better between the Holy See and the Palestinians.

Thus, it remains very unclear what organization, whether of the right or left, might be behind Agca's action. It is this very mystery that seems to be driving conspiracy-minded Italians to yet another hypothesis, namely that there is some terrorist center controlling Agca and on hire to some unnamed second group.

Italian officials are hoping a breakthrough in the mystery will come with the apprehension of two suspected accomplices, Mehmet Sener and Oral Gelik, both professional assassinss of the Turkish political strife like Agca.

Sener, according to these officials, was almost certainly in St. Peter's Square with Agca just prior to the shooting and was closely involved with him in the assassination of the Turkish editor.

But if Sener is as tough and cool as Agca in being interrogated, there is good reason to doubt that even his capture will lift the veil on the mystery of who was behind the attempt on the pope's life.