Experts Suspect Terrorists United to Bomb Airliner. Groups met in December to condemn Arafat, block US-PLO dialogue. FLIGHT 103: LOOKING FOR ANSWERS

AS experts search for clues while piecing together the jumbo jet destroyed last month over Scotland, specialists say the bombing may have been a joint operation by all or some of the suspect groups. These specialists point to indications that an unholy alliance among Palestinian and pro-Iranian radicals may be emerging out of the chaos in Lebanon.

The prime suspects to the bombing remain the same: some element of the Iranian government and/or their Lebanese Shiite allies, the Palestinian Front for the Liberation of Palestine-General Command (PFLP-GC), and the Abu Nidal Organization (ANO), also known as the Fatah Revolutionary Council.

Investigators are looking particularly carefully at the possible PFLP-GC link and the pro-Iranian path, informed officials say. But they are also keeping an eye on other possibilities, some of which could lead to even more obscure groups, such as the PFLP-Special Command, another Palestinian dissident group.

The United States, Britain, and West Germany are pressing hard for the full cooperation of a range of parties, including Syria and Czechoslovakia, believed to have manufactured the explosive used in the bombing.

The signs of a possible new terrorist coalition in Lebanon is attracting particular attention, even if it is not directly linked to the Dec. 21 bombing of Pan American Flight 103.

``What we're seeing is a realignment of forces in Lebanon to adjust to the new political realities in the Middle East. [PLO chairman Yasser] Arafat's decision to talk to the US and the end of the Iran-Iraq war have shaken things up,'' says a well-placed US specialist. ``The process is not unusual in Lebanon, but the new combination could be lethal because of the deadly histories of each of these groups.''

French terrorism expert Xavier Raufer says Palestinian hard-liners, including the ANO and the PFLP-GC, met in northern Lebanon last November. Their aim was to derail PLO leader Arafat's moves toward moderation and a dialogue with the United States.

Mr. Raufer says the Palestinian groups met Dec. 20 in Beruit with pro-Iranian groups, including several associated with the Shiite Hizbullah. This conference of ``rejectionists'' confirmed their support for the Palestinian uprising, condemned Arafat as a traitor, and threatened the US, according to Raufer's sources.

US specialists confirm that the Beruit meeting took place, apparently with some Iranian sponsorship. But they are unsure what, if anything concrete, was decided.

Nevertheless, these experts point to ample evidence of converging interests between these philosophically diverse terrorist groups. All oppose any lessening of the armed struggle with Israel, none are friends of the US, and all are feeling isolated.

Hizbullah is under sharp military pressure from Syria's Lebanese Shiite allies, Amal. Syria is not willing or able to stop the fighting. Reportedly, a number of Hizbullah's factions are alienated from the more moderate line taken by the majority in Tehran. These Lebanese are linked to more radical Iranians, such as the Revolutionary Guards.

ANO is a long-committed enemy of Arafat. Its record since 1973 includes more than 100 terrorist attacks. It has in recent years recruited Islamic fundamentalists for some of its operations and last year offered its services to Iran, according to well-placed Western intelligence sources. With political headquarters in Libya, ANO runs several military bases in Lebanon and maintains an international network of terrorist cells.

Well-informed Western specialists say operational cooperation among the ANO and elements of Hizbullah may already extend to recent kidnappings in Lebanon and attacks elsewhere on Saudi Arabian diplomats. The PFLP-GC is also a longtime opponent of Arafat.

But since Arafat's agreement to the US conditions for dialogue, the PFLP-GC has been more critical than has its main state benefactor, Syria. The PFLP-GC has adopted a much more fundamentalist tone in its language of late, which one US expert says ``appears to reflect unhappiness with being under Syria's thumb,'' and a cozying up to Islamic radicals.

Washington and others investigating Flight 103 are particularly eager for full cooperation from Syria. It has close ties to the PFLP-GC and other dissident Palestinian groups which would have an interest in disrupting the US-PLO dialogue.

Syria plays a major role in Lebanon, where the plot against Flight 103 may well have been hatched. And, despite recent tensions, it still maintains relatively close relations with Iran. Iran's Revolutionary Guards and secret services operate in Syria and Syrian-dominated Lebanon.

The PFLP-GC has its political headquarters in Syria's capital and its bases in Lebanon. It has long been funded by Syria and Libya, though it is more independent than several of the other Palestinian groups which Damascus backs.

Western investigators want Syria to vigorously investigate the PFLP-GC's operations in West Germany. Several operatives were arrested last fall with detonating devices and explosives for mid-air attacks and on suspicion of bombing US military trains.

German investigators originally thought the PFLP-GC operatives arrested were planning to blow up an Air Iberia jet flying to Israel. But as a US specialist notes, ``The Germans could easily have missed one PFLP cell, and that cell's target could have been the Pan Am jet.''

PFLP-GC leader Ahmad Jibril has denied responsibility for the attack, but in early 1986 he specifically threatened to attack US airliners. His group has hijacked and blown up US and Israeli airliners in the past. Top Syrian officials have accused Israel of inciting the attack on Flight 103.

``The motive here holds together completely,'' a ranking US specialist says. The only claim for the downing of Flight 103 was issued in the name of the Guardians of the Islamic Revolution - a cover name previously associated with Iranian special services, according to Western specialists. The caller claimed the attack as revenge for the US accidental downing of an Iranian passenger jet last July.

US specialists point out that though a bomb attack would be inconsistent with the more moderate line now being followed in Tehran, it could fit well into the plans of Iranians or Lebanese Shiites who want to keep the revolution and its anti-American flavor alive.

French specialist Raufer suggests the attack on Flight 103 may have been a joint operation, with a strong role by the ANO.

ANO has organized such joint ventures before, Raufer says. It provided supplies and logistics; others provided personnel.

ANO also has a well-developed terrorist network in Europe. Plus, Raufer's contacts say, the ANO picked up some of the expert bombmakers from the infamous May 15 Organization - a Palestinian terrorist group that broke up in the early 1980s. This cadre could have helped make the device that downed the plane.

US experts agree that some cooperation may have taken place. The trick will be to find the proof.