As soon as the shooting ended at the Rome and Vienna airports today, a deadly guessing game began. Who was the mastermind? What was the motive? Who will be held responsible by Israel and who will receive its retribution?

The conclusions drawn and the actions taken on them could well decide the course of the Middle East peace process at a moment when there is both hope for progress and a feeling that time is slipping away.

The horror unleashed on holiday tourists near the Israeli airline's counters comes on the eve of a summit next week -- the first in seven years -- between Jordan's King Hussein, who has been pressing a new peace initiative, and Syrian President Hafez Assad, the most wily opponent of any concessions to Israel.

It comes as Palestine Liberation Organization Chairman Yasser Arafat has been out of sight for days, ostensibly with health problems. And this latest terror attack adds to the series of episodes -- the TWA hijacking in June, the seizing of the Italian cruise ship Achille Lauro in October and the hijacking of an Egyptair jetliner in November -- that has kept a sort of grim pace with the hopes for new peace efforts.

What is known with any certainty about the terrorists at this point is almost nothing. They appeared to be Arabs, witnesses said. Some reports suggested that they might be Palestinians.

Many moderate Arabs and western analysts in the region say a new group of young men and women has grown up in the atmosphere of hatred and frustration aimed at Israel who are willing to do anything to strike at it. Some of them, in this view, are so numbed by violence that they can be recruited knowing little about the conspiracies that fund them and arm them and put them in place to act.

The consequences of the terror directed at Israeli targets often are meant to affect not only Israel, which has never been softened by terror, but those Arabs who have made an effort to find peace with Israel.

In today's attacks, as in previous ones, western analysts said, whoever planned it anticipated quick retribution from Jerusalem against someone. The Israeli raid against PLO headquarters in Tunis on Oct. 1 to avenge the killing of three Israelis in Cyprus made it clear that Israeli Prime Minister Shimon Peres would reach over long distances to demonstrate his country's refusal to bend.

The organization most vulnerable to Israel's retribution, and traditionally most likely to receive it, is the PLO.

Spokesmen for Arafat's organization were quick to deny any connection to today's attacks, and to condemn them. Since both Austria and Italy have been supportive of more moderate PLO initiatives in the past, the choice of airports would seem to be aimed intentionally at embarrassing Arafat.

But the PLO condemned the killings in Cyprus as well, despite Israeli claims that a special PLO unit was responsible. They condemned the Achille Lauro cruise ship hijacking in October, but later admitted that it was the result of an aborted operation launched by a PLO faction under Mohammed Abbas, a member of the Executive Committee. As a result, PLO credibility is low, even in the Arab world.

Within the international and Arab world politics that Arafat has been intensely involved in this year, however, the PLO leader would seem to have nothing to gain and an enormous amount to lose from such actions as today's.

Since the mid-1970s, Arafat has attempted to distance himself from the kind of spectacular international terrorism that such PLO-linked organizations as Black September made infamous at the 1972 Munich Olympics massacre.

In early November, under pressure from Jordan's King Hussein and Egyptian President Hosni Mubarak, Arafat, on a visit to Cairo, reiterated his stand against terrorism of this sort and pledged to limit military operations against Israel to the territories it occupies, which other Palestinian commanders have said means all of Israel.

Already badly weakened, with no firm base in Lebanon, slowly evacuating his remote headquarters in Tunisia, despised by radical Syria and Libya, kept at a distance by the cautious oil states of the Persian Gulf, Arafat would find himself with virtually nowhere to turn if Jordan and Egypt rejected him. This they certainly would do if he were linked to today's events after his Cairo declaration.

The Achille Lauro episode strained his relations with Egypt and Jordan almost to the point of rupture. Arafat's military second-in-command, Khalil Wazir, described it in a recent interview as "a political massacre to our cause." Another such fiasco would be the end of Arafat as peace partner, and perhaps to Arafat as leader of the PLO, in the view of moderate Arab leaders.

The PLO's erratic behavior in the past does not allow it to be ruled out as a suspect today. But the banners of radical violence also have been taken up by others.

The most notorious of Arafat's Palestinian opponents is Sabri Banna, also known as Abu Nidal, who broke off from Arafat's Fatah organization and the PLO in 1973 as Arafat began to seek international legitimacy. Since then, operating out of Iraq, then Syria and Libya, Abu Nidal and his agents have claimed responsibility for the slaying of numerous PLO and Palestinian officials thought to be willing to seek accommodation with Israel, as well as attacks on Jews and Israelis. The PLO has condemned him to death, but apparently never has been able to catch him.

It was Abu Nidal and Libyan leader Muammar Qaddafi whom Egypt blamed for the hijacking of its airliner last month, although little proof has been made public to substantiate the allegation and even the identity of the surviving hijacker in custody in Malta remains a matter of doubt.

Abu Nidal also claimed responsibility for the attempted murder of Israel's ambassador to London in 1982. Instead of retaliating against him, however, Israel used the attack to justify its invasion of Lebanon to destroy Arafat's organization there.

Putting aside other and murkier political ramifications, it is such precedents for retaliation by Israel that raise consternation among the moderate forces in Arab politics when an attack such as today's occurs. They fear that Israel will use any pretext to attack the PLO, even when the facts about precisely who carried out the raid are unclear, rather than let it represent the Palestinian people in negotiations.

The PLO's offices in Sudan have been warning for weeks that they suspect and fear an Israeli attack. Its political headquarters in Tunisia already has been decimated, but it reportedly has concentrations of guerrillas in Algeria and in North Yemen and Iraq.

The PLO also has military leaders in Jordan. Right-wing Israeli officials have warned of possible retaliation against them for actions against Israel. But any such reprisal also would be likely to end any chance of moving forward on the peace process that Prime Minister Peres appears to want.

Although Hussein's relations with Arafat remain strained, few of his people would blame the PLO if Jordan were attacked by Israel.

The king reportedly hopes to bring Syria into the peace process at the upcoming summit, which was scheduled for Saturday but postponed until next week following today's attacks. Assad reportedly is intent on pulling the king away from Israel, not moving with him toward it.

Syria is closely linked to Abu Nidal and several other dissident Palestinian factions capable of carrying out today's attacks in Rome and Vienna. It also supports Shiite Moslem resistance movements in southern Lebanon and gives such groups training and base facilities in Lebanon's Bekaa Valley and other places.

But if Syria was behind today's attack, President Assad would be making a big gamble. Already his movement of antiaircraft missiles into the Bekaa, then out, then back, in a matter of days has provoked a sharp confrontation with Jerusalem. Support for a terrorist action now would push tensions further than Assad has cared to do since Israel last destroyed his Air Force in 1982.

Because of Libyan leader Qaddafi's known links to terrorist actions in the past, his suspected links to the Malta hijacking, his dislike for Arafat and his close association with Abu Nidal, he is also a potential target for reprisal.

Finally, there are Iraq and Iran. Although they are at war with each other, both also are implacably opposed to Israel and both have supported terrorist groups acting against it and the United States. Iraq has supported Arafat's PLO recently, and earlier backed Abu Nidal. Iran, allied to Syria and Libya, is believed to have abetted a number of the region's terrorist actions since 1979.