Two main lines of inquiry are being explored by Middle East analysts trying to pin down responsibility for the bombing of Pan Am Flight 103. One is that Palestinian extremists - possibly backed by an Arab state - were trying to disrupt the newborn dialogue between the United States and the Palestine Liberation Organization (PLO).
The other is that Iranian-inspired Muslim radicals were seeking revenge for the downing of an Iranian Airbus by US forces in the Gulf last July.
But there are flaws in either theory.
In the case of the Palestinian hard-liners - or anybody else trying to discredit the PLO and derail its diplomatic peace drive - the objective would clearly have been somehow to implicate Yasser Arafat's mainstream organization.
That would normally be done by one or more bogus telephone calls to news agencies claiming responsibility for the bombing on behalf of some fabricated, Palestinian-sounding and hitherto-unknown group.
The only telephone claim of responsibility so far was made in London by the ``Guardians of the Islamic Revolution,'' and that it was done to avenge the downing of the Iranian airliner. The suggestion that the action had been sponsored by Tehran was immediately denied by Iranian leaders.
Iranian-inspired Muslim extremists have carried out a long series of actions classified as international terrorism, including, most recently, the hijacking of a Kuwaiti airliner last April and the murder of two of its passengers.
But they have almost always had a fairly clear political objective, and have rarely involved in purely vindictive acts of revenge, analysts say.
The Iranian-backed radicals - notably the Lebanese Hizbullah and related Shiite networks - would, however, almost certainly have had the expertise to plant such a bomb, experts believe.
As for political motivation - apart from revenge - a case might be made that radical factions in Tehran might have sponsored the operation in order to embarrass and undermine more moderate or pragmatic elements of the Islamic regime.
Such often apparently senseless acts of violence have, however, more often been the hallmark of the extremist Palestinian faction headed by Sabri al-Banna, better known by the name Abu Nidal.
While the considerations outlined above may argue against the involvement of Palestinian radicals, observers point out that Abu Nidal's Fatah Revolutionary Council is the prime suspect in three terrorist attacks this year which also had little apparent logic behind them. They were a car bomb explosion in Nicosia, a grenade attack on expatriate clubs in Khartoum, and the attack on the Greek cruise ship, City of Poros, near Athens.
Abu Nidal's track record alone is enough to make him a prime suspect in the Palestinian arena. His followers are thought to have carried out at least 60 acts of violence in 20 countries.
In 1973, Abu Nidal broke with Arafat's Al-Fatah faction, the PLO's largest, in protest at its willingness to negotiate peace. Since then, Abu Nidal's operations have been directed solely at sabotaging the PLO's policies.
At various times in the past, Abu Nidal has worked closely with Iraqi, Syrian and, most recently, Libyan intelligence, Arab and Western experts say.
The Arab state most bitterly opposed to the PLO's current diplomatic drive, Syria, was strongly implicated in an attempt to smuggle a bomb on board an El Al airliner at London's Heathrow Airport in 1986.
But since then, Syria has ostensibly broken with Abu Nidal and expelled his offices from Damascus. It has made efforts to improve its international image, and succeeded in persuading the US - but not Britain - to send its ambassador back.
Over the Pan Am affair, some of Syria's regional adversaries, including hard-line Lebanese Christians, have pointed an accusing finger at Damascus and the Palestinian factions closest to it, such as Ahmad Jibril's Popular Front for the Liberation of Palestine-General Command and Abu Musa's Fatah-Uprising. But neither of those factions has a track record in international terrorism.
Some observers also strongly doubt that Syria, currently in acute political isolation, would risk the tremendous damage that association with such a terrorist outrage would entail.
``It doesn't make sense, especially after they got their fingers so badly burned over the Hindawi [Heathrow] affair,'' says one source.
Israeli intelligence officials believe that two smaller, radical Palestinian groups have carried out Abu Nidal-style terrorist attempts in past years. They are the ``15th of May Arab Organization'' led by Muhammad Amri (codename Abu Ibrahim), and the PFLP-Special Command, headed by Selim Abu Salem. However, their last known bombing attempt is thought to have been in 1984.
``None of the suspects can be ruled out, but there's no public evidence so far pointing conclusively at any of them,'' a Middle East analyst says.
``It's a wide open case right now,'' he adds.