FBI officials investigating the explosion aboard a TWA jet en route from Rome to Athens Wednesday now are focusing their efforts in Cairo, where a woman sought in the case had boarded the same aircraft earlier that day, a senior TWA official said today.

U.S. government and TWA officials said that the woman identified as May Elias Mansur -- whose name and Lebanese passport they acknowledged may have been false -- remains their central lead thus far. But there was no indication that any direct link had been established between her and any known terrorist group that might have perpetrated the bombing, which killed four persons.

In Washington, administration officials said U.S. diplomats and facilities, especially in Western Europe and the Middle East, were on an unusually high state of alert because of continuing reports that they were targets for attacks by groups encouraged by Libya, Washington Post staff writer Don Oberdorfer reported.

Law enforcement officials in Washington also said that they had no leads in the TWA bombing other than Mansur and that they were taking a skeptical approach to the claim of responsibility made in Beirut by a previously unknown group using the name Arab Revolutionary Cells.

In Tripoli, Lebanon, a statement issued Friday in the name of May Elias Mansur said she "had nothing to do with" the TWA bombing and threatened to sue everyone who had "falsely accused" her. There was no way to authenticate the statement, which was distributed by an unidentified group.

Charles Adams, vice president for TWA's international division, said that Greek and U.S. officials had completed their inspection of the Boeing 727 and released the aircraft for repair. The explosion aboard Flight 840 blew a 4-by-3-foot hole in the side of the plane. There was no damage to operational systems, and Adams said that initial repairs in Athens would enable the aircraft to be flown back to TWA facilities in Kansas City.

Adams said that the seat in the passenger cabin under which the bomb had been placed was still in the custody of Greek security. FBI and U.S. Federal Aviation Administration officials who have been permitted to examine it were able to make preliminary conclusions as to the position of the bomb, which is believed to have been made of a plastic explosive. But without the seat, it remained difficult to determine with certainty the type of explosive or the detonating or timing devices.

The Greek government, while described by Americans as cooperative, has been firm in its insistence that it is in charge of the investigation. In a news conference today, Foreign Minister Karolos Papoulias described FBI, FAA and TWA investigators, along with an Italian security team, as having "observer status."

Despite repeated questioning, Papoulias declined to provide any details of his government's investigation. "It is too premature," he said, and advised patience "to allow the police to continue . . . without interfering with premature judgments."

Papoulias also used the occasion to repeat his government's "vehement" rejection of terrorism. He called for "a stop to the arbitrary denunciation of Greece" as a country where terrorism, particularly at the airport in Athens, is likely to occur.

Describing a "state of hysteria" created erroneously, Papoulias said that the "Greek airport has been judged by international experts as being absolutely safe." U.S. Secretary of State George P. Shultz had declared it so during his visit last week, he said.

Officials close to the investigation here have cautioned that evidence connecting Mansur to the bombing is largely circumstantial. Some have expressed concern that concentration on her may shut out investigation of other possible leads.

But indications here today were that there are no other substantial leads. The FBI, one TWA official said, "has been pretty straightforward that this is the best suspect . . . . They are seeming to focus on her."

The main investigation has moved to Cairo, another airline official said, both because Mansur boarded the airplane there and "because she spent a lot of time there," between arriving March 25 and her departure Wednesday.

The bombed Boeing 727 visited three cities on Wednesday -- Cairo, Rome and Athens -- and the explosive theoretically could have been placed aboard at any of them.

The explosion, apparently released by a timing device, took place on the Rome-Athens leg of the circuit, as Flight 840 was beginning its descent into Athens for the second time that day.

The first allegation that someone named Mansur was involved came as a result of what apparently was a case of mistaken identity by Italian security officials. Obtaining a list of all passengers who had been aboard the plane Wednesday, they found a listing for M.E. Mansur traveling from Cairo to Athens. According to sources in Rome, they recognized the surname as that of a male they believed responsible for placing a bomb aboard an Alitalia flight in 1983. That bomb was found in a suitcase before it went off. Mansur -- or Mansuri by some accounts -- was never apprehended.

Italian officials subsequently were quoted as saying that a "known terrorist" had been aboard the TWA plane. ANSA, the Italian news agency, quoted unnamed security sources as saying that Mansur was an "expert in explosives."

Later yesterday, however, it was confirmed here and in Cairo that the Mansur on the TWA passenger list was a woman. Even though she was not the person originally identified by the Italians, a number of things about her already had raised questions in both cities.

Most importantly, Mansur had been seated in seat 10F during the Cairo-Athens flight. This is the same seat under which authorities now believe the bomb was placed. When it exploded, approximately six or seven hours later, it blew a hole in the side of the aircraft through which the seat and its last occupant, Alberto Ospina, were sucked out, along with two women and an infant who had been in the row behind him.

TWA and Egyptian authorities in Cairo then said that Mansur, who had traveled on a Lebanese passport, had attracted attention for several reasons before boarding the plane. The last person to arrive for the flight, about 35 minutes before takeoff scheduled for 5:40 a.m., she missed the last boarding bus and had to be driven to the aircraft by a TWA official.

During the flight, Mansur, who has been described as about 30 years old, five feet tall, dark-haired and possibly walking with a limp, was quiet. By one account, she listened to a cassette tape player through earphones.

The airplane, with a capacity of 140 passengers, was nearly empty from Cairo to Athens. Of the 17 persons aboard, in addition to Mansur there were eight Americans, one Canadian, two Australians, four Egyptians and one Somali. Although Mansur got off in Athens, Greek immigration and customs have no record of her leaving the airport.

Three, perhaps four, TWA employes later reported having seen her waiting in the transit lounge, airline officials said. At about 3 p.m. she boarded a Middle East Airlines flight for Beirut. That means it is likely she was in the Athens transit lounge when the bomb exploded on Flight 840 at 2:20, and was still there when the stricken plane landed here at 2:33.

Despite Wednesday night's statement of responsibility, made by an anonymous telephone caller in Beirut, little is known about the Arab Revolutionary Cells.

Some U.S. news reports, and a broadcast today by the Christian Voice of Lebanon radio, alleged that Mansur belongs to an organization variously known as the Lebanese Revolutionary Brigade, or Lebanese Armed Revolutionary Faction. A Marxist Christian group, the faction is believed by French authorities to be responsible for the 1982 assassination of a U.S. military attache in Paris. One of the group's leaders is being held in a French prison for the crime, and some of the recent bombings there have been attributed to faction members demanding his release.

The Lebanese Armed Revolutionary Faction is based in northeastern Lebanon, north of Tripoli. On the landing card she filled out when she entered Cairo on March 25, Mansur identified Tripoli as her birthplace.

In Washington, one senior official said other theories are that the bombing may have been the work of two radical Palestinian groups: the May 15 organization or a Syrian-backed radical group headed by Abu Musa that broke away from Yasser Arafat's mainstream Fatah organization and is based in the Bekaa Valley of eastern Lebanon.

TWA officials said that the airline now is most interested in finding out how the bomb passed through what it describes as stringent security procedures in Cairo, Athens and Rome.

Airline officials responded sharply this afternoon to morning broadcasts from London by the British Broadcasting Corp. in which an unnamed TWA official was quoted as admitting that a "lapse" in the airline's Cairo security procedures had allowed the bomb to pass through.

In a lengthy statement distributed here, TWA said that it has "completed a full and comprehensive review of all security procedures in effect on April 2 at Rome, Athens and Cairo. No breaches in that security have been found."

The statement said TWA had paid "particular attention" to security procedures enforced in Cairo on Wednesday. Mansur, it said, had been subjected to "the most rigorous" checks, including an X-ray of her carry-on baggage, two searches of her checked baggage and a search of her person and carry-on baggage by Egyptian airport officials.

TWA noted that while it had concluded that all airline and government procedures had been met, "Egyptian authorities gave final approval for her to leave the terminal."