Washington — Finding the group that attacked the American marines and French paratroops in Beirut may prove to be an impossible task. Even if the Reagan administration is able to identify the group, it will make its task of dealing with the larger problems of Lebanon no easier.
Administration officials claim to have circumstantial evidence that a group with ties to Iran may have been involved. The suicide attacks had all the marks of the earlier attack on the United States Embassy in Beirut. Pro-Iran extremists were suspected in that attack. Iranian Muslims are certainly capable of fanaticism and suicide attacks.
But beyond such vague circumstantial evidence, the administration apparently has little to go on.
In an appearance before editors and publishers in Washington on Monday, President Reagan called it ''strong circumstantial evidence,'' but some experts are describing the evidence as far from hard.
The President on Monday reasserted that the United States has ''vital'' interests in Lebanon, and that Lebanon is the key to the entire region's stability as well as to America's global credibility. An American withdrawal would be a ''disaster'' for the region and the world, Reagan said. But he ruled out an increase in the marines' presence and role. He said that to assume a more aggressive posture would move the US beyond a peacekeeping role and into the position of ''fighting Arab states.'' This in turn would not only defeat the administration's broader goal of an Arab-Israeli peace but would also risk world war, the President said.
The attack on the US marines dramatizes the administration's dilemma in dealing with Syria. It must negotiate with Syria in order to secure its goals: Lebanese stability and a withdrawal of foreign forces, including both the Syrians and Israelis. But the administration has identified the Syrians as the main obstacle to those goals. The pro-Iranian group suspected of attacking the Americans and the French is also believed to operate from behind Syrian lines.
A final problem: Even if this group is positively identified as an organizer of the attacks, it is by no means certain that it will be the sole group responsible for the attack. There appear to be several factions in Lebanon that would like to force out the Americans and French.
This does not mean that all the Muslim groups in Lebanon oppose the US and French presence there. At a breakfast meeting organized by Foreign Policy magazine on Monday, Sayed Hussein Husseini, a founder of the Shiite Higher Islamic Council, described the US marine presence in Lebanon as positive. Mr. Husseini, a leader of the Shiite paramilitary and political organization Amal until he resigned in 1980, was in Washington at the invitation of the international visitor program of the US Information Agency.
In his remarks, President Reagan described the group or groups that attacked the marines and French paratroops as a small minority who were not part of the main warring factions and were little more than ''criminals and thugs.''
''The tragedy is coming not really from the warring forces,'' said Reagan. ''It is coming from little bands of individuals, literally criminal-minded, who now see in disorder which is going on an opportunity to do what they want to do.''
In an anonymous telephone call to a Beirut-based news agency Monday, a group calling itself the Islamic Holy War claimed responsibility for the suicide bombings in Beirut. But Iran's Islamic regime on Monday dismissed suggestions made by US officials that Iran might be behind the bombings.
Joseph Sisco, a former US undersecretary of state and Middle East negotiator, said Monday that three significant forces among Lebanon's 17-odd main factions want to see the US forced out. He said that one of these, the Lebanese National Resistance Front, had been responsible for inflicting many of the casualties the Israelis suffered in Lebanon.
Dr. Sisco recommended getting key West European nations more deeply involved in efforts aimed at achieving a reconciliation in Lebanon. The peace process should be broadened, Sisco said, to include, among other things, negotiations over the Israeli-occupied Golan Heights. The US should make informal contact with the Soviet Union, said Sisco. Finally, he recommended that a substitute multinational force be prepared to take over from the American marines and French paratroops. This, he said, could include the Greek and Italian troops who have already been agreed upon as Lebanon observers as well as troops from Saudi Arabia and the Scandinavian nations.
Sisco said that the problem with the Americans and French at the moment was that the Americans were seen to have tilted in favor of the Maronite Catholics in Lebanon, while the French had tilted toward Iraq. The Americans and French had therefore become ''targets'' rather than peacekeepers.
Sisco disagreed with former Secretary of State Henry A. Kissinger, who has proposed that the US shift the balance of power in Lebanon against Syria by bringing Israeli forces back into Lebanon action.