Working with unusual urgency, experts at a British army ordnance laboratory in Kent took only days to determine the cause of the crash. From wreckage recovered near the devastated rural town of Lockerbie, they examined a ripped suitcase, fabric from some passenger seats and fragments from a metal bin in which checked luggage was packed and then rolled into the cargo hold of the Pan Am 747 at London's Heathrow Airport. Two pieces of the container's framework were pitted and showed other signs that a ''high-performance plastic explosive'' had erupted near them. Scotland Yard's antiterrorism branch and the FBI jointly assumed the difficult task of finding out how the bomb got on the plane.

   Engineers at Seattle's Boeing Co., makers of the 747, said the explosive almost certainly had been placed in the aircraft's forward baggage hold, just in front of the section where the wings are attached to the fuselage. They estimated that about 10 lbs. of a plastic explosive had in effect decapitated the 747, instantly severing the cockpit and part of the first-class cabin from the rest of the plane. Because the forward luggage compartment is next to the main electronics bay, the explosion instantaneously cut off all communications, electricity and flight controls, explaining why all systems  went dead at the same moment. Declared a Boeing expert: ''It was a diabolically well-planned event, handled by experts in knowledge of the aircraft, its structure, the flight plan -- the works.''
 
 The bomber's only mistake apparently was in timing. Terrorism experts assume that a timer had been set so that the charge would explode after the flight cleared the British Isles and was over water on its course to New York. If so, specific evidence of the sabotage would have been almost impossible to dredge up from the wintry Atlantic. But Flight 103 left Heathrow 25 minutes late. Anticipating such delays, terrorists have used barometers to start a timer only when a set air pressure has developed near the bomb. Since the cargo holds in a 747 are pressurized after takeoff along with the cabin, the barometer could detect this change and start the timer. If such a technique was used on Flight 103, it failed to postpone the blast until the aircraft was over water only because high-altitude winds caused the crew to take a northerly course over Scotland before
heading west.
 
 Who has this kind of expertise on explosives? No one is jumping to quick conclusions. But Palestinian sources, as well as some in the U.S. Government and Israeli intelligence, probably the world's best trackers of terrorist groups, point to Ahmed Jibril, leader of the Damascus-based Popular Front for the Liberation of Palestine-General Command. Fourteen members of Jibril's group, which fiercely opposes P.L.O. chairman Yasser Arafat's decision to recognize Israel's right to exist and open talks with the U.S., were arrested by West German authorities in October. Seized with them was a cache of arms thatincluded the ultimate boom box: a portable radio packed with a plastic explosive so cleverly concealed that the radio still worked.
The wire detonator was fashioned to look under X rays like the radio's antenna. Israelis say the group had planned to blow up an Iberia Airlines flight carrying tourists to Israel.
 
 The Jibril terrorists have a history of aerial bombings. They claimed responsibility for the 1970 explosion that downed a Swissair flight shortly after takeoff from Zurich on its way to Tel Aviv, killing 47 people, and for a 1972 blast aboard an El Al airliner that landed without casualties. West German police are searching for any connection between this group and the Pan Am tragedy. ''The group is pro-Syrian, anti-Arafat and anti-P.L.O.,'' contends a U.S. State Department fact sheet. ''It has strong ties to Syria, although Libya has also long supported it.''
 
 Another suspect is Abu Nidal, the fanatic P.L.O. terrorist whose Fatah Revolutionary Council allegedly carried out the 1985 Christmas massacres at the Rome and Vienna airports. He too would like to scuttle Arafat's Middle East peace moves. ''Such an act of terrorism by Abu Nidal would be a message to the U.S. and a slap in the face for Yasser Arafat,'' said Ian Geldard,   director of research at London's Institute for the Study of Terrorism. Allied with Libya, Abu Nidal would presumably have access to Muammar Gaddafi's ample supply of Semtex, a plastic explosive made in Czechoslovakia.

   One member of Abu Nidal's P.L.O. faction is, in fact, already charged with a plane bombing. Greece is holding Mohammed Rashid on false passport charges while deciding whether to extradite him to the U.S., where he is wanted for the 1982 explosion aboard a Pan Am flight from Tokyo to Honolulu. The pilot landed in Hawaii with 285 passengers, but a 16-year-old Japanese boy, seated close to the exploding bomb, was killed.

   Still, Israeli intelligence places Abu Nidal well behind Jibril as a Flight 103 suspect. ''Abu Nidal certainly wants to undermine Arafat and do a favor to his sponsors, the Libyans, helping them take revenge on the Americans,'' says one Israeli expert. ''But he has no expertise in this type of action. His specialty is assassinations.''
While a caller to the U.S. embassy in Helsinki had warned that terrorists allied with Abu Nidal planned to sabotage a Frankfurt-to-New York Pan Am flight, Finnish authorities insist that the tipster was a habitual alarmist whose call was a mere
coincidence.  Said FBI director William Sessions last week: ''The bureau believes that it was a hoax and not connected to Flight 103.''

Various Iranian fundamentalist factions have claimed that they arranged the Pan Am bombing to retaliate for the U.S. Navy's accidental downing of an Iranian airliner over the Persian Gulf last July in which 290 people died. Intelligence sources generally doubt these groups have the required technical knowledge to carry off such an operation. Whoever executed the deadly deed was probably targeting the jumbo jet's large contingent of American passengers heading home for the holidays rather than individual travelers.
While the CIA flatly denied reports that its Beirut station chief was a passenger, two regional State Department security officers, as well as a U.S. diplomat assigned to the Beirut embassy, were on board. But investigators think it implausible that anyone wanting to kill known CIA operatives would try to follow their uncertain travel plans rather than plot an ambush where they work. A bit belatedly, the Federal Aviation Administration ordered U.S. airlines to take new security measures on flights at 103 airports in Western Europe and the Middle East. They include either X-ray or physical inspection of all % checked luggage, random opening of carry-on baggage, and a ''positive match'' of passengers and bags to make sure that no suitcase is loaded on a plane without its owner taking the same flight.

The need for tougher measures was apparent long before the metallic shower struck Lockerbie.
A ten-man Israeli security team studied 25 Pan Am airport facilities for the airline in 1986. It concluded that Pan Am was ''almost totally vulnerable to midair explosion through explosive charges concealed in the cargo.'' The team claimed, for example, that baggage could be loaded on Pan Am airliners in London and Hamburg without its owner also boarding; that Pan Am planes too often carried both passengers and general cargo; that in Europe the checked baggage of some citizens of certain nations, including the U.S., was not examined at all. The Israelis say Pan Am officials rejected many of the suggested remedies as too expensive for such a large airline to implement. ''We told them many times it was a matter of life and death,'' said one of the authors of the report last week. ''But they seemed to know better and told us they would go their own way. What a pity.''

Ed Magnuson. Reported by Ron Ben-Yishai/Jerusalem, Jerry Hannifin/Cape Canaveral and Christopher Ogden/London, NATION: ''Diabolically Well-Planned'' The hunt begins for the bombers who doomed Pan Am's Christmas Flight 103., TIME, 01-09-1989, pp 26.