About one pound of a standard type of plastic explosive, and a detonating and timing device so small it might escape recognition during an X-ray, were used to fashion the bomb that exploded aboard TWA Flight 840 last week, the FBI has preliminarily concluded.

Sources close to the investigation here said that a Lebanese woman who had boarded the aircraft in Cairo on the day of the explosion remained the focus of suspicion. But they said it was conceivable that the bomb could have been put in place days or even weeks before, with its timer set for a specific time.

The Boeing 727 aircraft had been flying a regular circuit among the Cairo, Athens and Rome airports for some time.

In another development, U.S. Federal Aviation Administration investigators who arrived here after the bombing have approved Athens' airport security operations. Without ruling out Athens as the place where the bomb was brought aboard the plane, sources said, the FAA was "satisfied" with security procedures and their implementation.

Authorities now have concluded most of the technical aspects of the investigation, which is being conducted under the auspices of the Greek police. Additional information about the explosive will be gleaned from autopsies to be performed today on the bodies of the four passengers who were sucked through the hole blown in the side of the aircraft. U.S. military pathologists have flown here from Washington to attend the examination, which reportedly will be carried out by Greek officials.

Most important to their efforts is the body of Alberto Ospina, who was sitting in seat 10F on the airplane. The bomb was located below that seat, although TWA spokesman Steve Heckscher said it was planted under the seat cushion, above the frame and above the under-seat life jacket, not below it as earlier supposed. Thus, neither the required visual search nor the random hand search beneath the seat would have detected the explosive.

The plastic explosive is a putty-like substance that can be molded into any shape. It is nonmetallic, lightweight, and will not explode unless detonated. The volume of a pound of the substance is equivalent to that of two or three cigarette packages.

Sources familiar with similar investigations displayed some pessimism today that the perpetrator of the bombing would be apprehended or even positively identified. "My experience with these things is that there will be nothing absolutely conclusive," said one source.

FBI investigators now in Cairo are trying to trace the activities of a woman carrying a Lebanese passport in the name of May Elias Manssour, who arrived in Cairo March 25, and left there early Wednesday on TWA's flight to Athens. She apparently waited in the Athens airport before boarding a Middle East Airlines flight to Beirut. On the Cairo-Athens flight, Manssour was reported to have occupied seat 10F, where the bomb exploded later that day as the plane was carrying a different group of passengers from Rome to Athens.

Although they have pinpointed no groups for responsibility, officials have mentioned several terrorist organizations since the bombing. On Wednesday and Thursday, anonymous communications in Beirut alleged that the little-known Arab Revolutionary Cells had placed the bomb. Other unconfirmed accounts have tied the bombing to Abu Musa, the leader of a Syrian-backed breakaway group from the Palestine Liberation Organization; the Syrian- and Libyan-backed terrorist known as Abu Nidal; and a terrorist organization of Lebanese Marxist Christians called the Lebanese Armed Revolutionary Faction, believed responsible for a series of attacks in France.

One group known to have used a similar bombing style is the May 15 organization, another Palestinian splinter group headed by a person known as Abu Ibrahim. After a 1982 explosion that killed a Japanese boy aboard a Pan Am jet as it was beginning its descent into Honolulu, it was reported that May 15 had developed a bomb that could escape X-ray detection.

In 1984, members of the organization reportedly tricked passengers on flights from Tel Aviv into carrying bomb-rigged suitcases. Two alleged May 15 members were arrested last October in Rome for traveling with a similar suitcase bomb.

Aviation security sources said today that the level of terrorist sophistication, and the existence of minute timing and detonation devices, had made the task of detecting explosives ever more difficult.

Athens has been perhaps the most scrutinized airport in the world since terrorists who apparently boarded a TWA jet here hijacked it to Beirut last June. Initially, the U.S. government recommended that Americans not fly here, an advisory that was rescinded a month later.

In recent months, FAA investigators have made "regular" inspections of the airport, the source said, "both announced and unannounced." The most recent inspection -- including interviews with all airlines operating out of Athens and airport staff -- before last week took place less than one month ago.

In addition to near constant visual observation by airline and airport officials through the departure terminal, passengers boarding a flight here go through two passport checks and a metal detector, plus X-ray and personal examination of hand baggage by Greek authorities. At boarding gates, a number of airlines -- including the two American carriers that fly here, TWA and Pan American -- use their own metal detectors and conduct their own additional X-ray and personal inspection of hand baggage. The two American airlines also X-ray all checked baggage themselves.

The Athens airport authority has increased its staff and purchased new X-ray equipment from West Germany, an informed source said. "The security checks done in this airport aren't done in any airport in the United States," he said. "We don't have any better."