Airline and security officials said today that a woman traveling on a Lebanese passport is being sought in connection with yesterday's explosion aboard TWA Flight 840 and confirmed that the blast had been caused by a bomb under a passenger seat.

The woman had boarded the plane in Cairo and left it in Athens on the first leg of its round trip to Rome.

In Rome, Italian Interior Minister Oscar Scalfaro said the "suspect," whom he did not name, was "known as a terrorist" and had sat in "exactly the place where the explosion occurred." Italian police sources called the suspect an "expert in explosives."

Although authorities here and in Cairo said they could not verify Scalfaro's description of her terrorist history yet, they identified the woman as May Elias Mansur, born in 1955 in Tripoli, Lebanon.

They said that according to immigration documents in Egypt, Mansur arrived in Cairo on March 25 and departed early yesterday on TWA Flight 841 for Athens. After she got off in Athens, the plane, a Boeing 727, continued to Rome, where it picked up passengers from Los Angeles, New York and Rome before returning to Athens as Flight 840.

TWA officials here said that Mansur had occupied seat 10F on the plane's Cairo-Athens journey. During the fateful Rome-Athens leg later in the day, 10F was occupied by Alberto Ospina, a Colombian-born American. It was under that seat, officials said, that the bomb had been placed.

When the explosive detonated as the plane began its descent into Athens, the seat was torn out of the aircraft. It and Ospina were sucked immediately through a 4-by-3-foot hole in the side of the plane, along with Demetra Stylianopolous, 52, her daughter Maria Klug, 24, and granddaughter Demetra Klug, 9 months. All were U.S. citizens. Stylianopolous and the Klugs had been in row 11, behind Ospina.

TWA officials, who have been working here with U.S. teams flown in from the FBI and Federal Aviation Administration, as well as with Greek and Italian authorities, said it was not until today that they were absolutely certain that a bomb had caused the explosion. Greek officials had barred them and U.S. government investigators from boarding the plane until this morning.

Initial observation confirmed that the explosive device had been placed in the passenger cabin, rather than in the baggage hold. Exploding near the floor, it had torn a downward hole through the right side of the aircraft.

"Where the device was is completely obliterated," said Charles Adams, vice president for TWA's international division.

Adams said inspection of the plane was continuing. Seat 10F, recovered in the same area as the bodies of the dead, near Argos, southwest of Athens, was still being held by Greek authorities, he said. When U.S. officials have had a chance to examine it "in a few days," Adams said, they should be able to determine the type of explosive used and its exact positioning.

Responsibility for the bombing was claimed in an anonymous telephone call to a news agency in Beirut last night that attributed it to a group called the Ezzedine Kassam unit of the Arab Revolutionary Cells, named after a Palestinian who fought and was killed by the British during their occupation of Palestine in the 1930s.

It was not known if Mansur has any connection to the group. Although Adams confirmed that she was being sought as a principal lead in the case, neither he nor U.S. government officials here were prepared today to label her a suspect or to say that this was the only line of investigation.

"She may be a nice person and have five kids," Adams said. "We haven't gotten a lot of the information yet. It takes time."

[Rome airport sources told Reuter that Italian investigators believe Mansur may be the same person sought for trying to plant a bomb on an Alitalia flight from Istanbul to Rome in December 1983. The explosive, hidden inside a pack of cigarettes, was found inside a checked suitcase after its owner failed to board the flight, the sources said.]

U.S., Greek and Egyptian officials detailed a pattern of behavior that appeared to have made Mansur the focus of the investigation thus far. They said that her activities yesterday had attracted attention at several points.

TWA officials had noticed that she was the last person to check in for the Cairo-Athens flight, something they said always causes suspicion. After checking two large suitcases and carrying away a handbag, Mansur was the last person to arrive at planeside to identify her checked luggage before it was loaded aboard the aircraft.

Greek officials confirmed that there is no record of her passing through Immigration or Customs here or leaving the airport. They said they believed she had remained in the airport transit lounge from the time TWA Flight 841 arrived from Cairo at 8:35 a.m. until she left aboard a Middle East Airlines flight to Beirut.

An MEA flight to Beirut departed here yesterday at 4 p.m. No indication was given as to what happened to Mansur's checked baggage.

TWA Flight 841 carried 17 passengers from Cairo to Athens. Sixteen reboarded the flight to continue to Rome, along with a group of new passengers, including a family of five seated in row 10. Officials said there were no indications that the family had any connection to the bombing. They did not report anything suspicious found around seat 10F.

In Rome, the crew of the aircraft was changed before it began its return trip to Athens. The crew aboard the Cairo-Athens-Rome portions of the 727's journey have since traveled to New York, where TWA officials said they are being debriefed by U.S. officials, who can be expected to question them about all passengers aboard the plane, particularly the woman in seat 10F from Cairo to Athens.

One American official here speculated today that X-ray-proof plastic explosive could have been affixed to the terrorist's body in such a way as to avoid detection during a cursory frisking. But, he acknowledged, that does not explain how a detonator, presumably of metal, or a timing device, if one was used, could have been brought aboard.

TWA's Adams said that no one was allowed aboard the plane at any of its stops unless airline personnel or contracted security guards were on board, and all were required to pass through metal detectors. The plane was guarded, TWA officials said, during its overnight stay in Cairo.

In Rome, where its crew was changed, the aircraft was cleaned by security-checked personnel. When the new crew boarded, they conducted a standard search that included checking the floor underneath each seat to make sure nothing had been left there. Crew members said today they also conducted a "spot check" of below-seat life jackets, which involved reaching under seats at random and making sure the jackets were in place.

Investigators said they had ascertained that the explosive probably was affixed to the bottom of seat 10F, off the floor and attached below the life jacket. Crew members said today they did not recall whether seat 10F had been included in their spot check.

Not surprisingly, airport security officials in Cairo, Athens and Rome -- each of which has been victimized by terrorist attacks in the recent past -- were eager to defend their security procedures.

TWA Capt. Richard F. Peterson, who was piloting Flight 840, today described security at Cairo airport as the "tightest it's ever been" in the wake of a hijacking to Malta last November of an EgyptAir jet bound from Athens to Cairo. Security officials in Cairo said today that passengers and their luggage must go through five separate security checks before boarding an aircraft.

Although at least two of those checks involved a simple visual once-over by security and airline personnel, all passengers and their baggage are X-rayed -- twice in the case of hand baggage -- and passengers are given a "pat down" frisk. Outside the plane, all checked baggage must be identified by passengers before it is loaded.

Athens has been particularly sensitive to charges of lax security since the hijacking of a TWA passenger jet here to Beirut last June. U.S. officials advised American citizens last summer not to travel here by air, but later rescinded the warning. On a visit here last week, Secretary of State George P. Shultz pronounced Athens airport safe.

TWA canceled its Rome-Athens-Cairo flights until further notice. Today, an armed guard sat behind the empty TWA counter at Athens airport. Downtown, the main TWA office was patrolled by security police inside and outside the building.