Investigation of TWA blast focuses on how it happened. Officials hope efforts will bring new preventive steps

Although significant headway has been made in efforts to uncover the facts behind the explosion on a Trans World Airlines jet en route from Rome to Athens last Wednesday, several questions remain unsolved. The most complex one facing investigators is: After all the security precautions at both Rome and Athens aiports, how could such a terrorist incident occur?

The key objectives of the current investigation, local and foreign observers here say, must be to learn as much as possible about how the bombing was carried out, in order to improve chances of identifying and stopping such attempts in the future.

Investigators are running into difficulty substantiating their claim that May Elias Mansur, a Lebanese woman, planted the bomb on an earlier leg of the flight between Cairo and Athens. (The plane then flew to Rome and the explosion occurred as it made the return trip to Athens.) On Thursday, officials involved in the investigation had indicated that Mrs. Mansur was a known terrorist. But by the weekend, Italian and United States officials said no evidence has emerged that links Mansur to a known terrorist group.

In Tripoli, Lebanon, Mansur denied Saturday that she had anything to do with the bombing. Although she acknowledged that she was on the flight from Cairo to Athens and later flew on to Beirut, she denounced the bombing because it claimed innocent civilian lives. ``I support attacks against American targets, but not the way this attack was carried out,'' Mansur said.

However, a Greek police official said Sunday, ``We are 95 percent certain it is her.'' If ``she had no involvement in the attack as she says,'' the police official added, ``let her come here to explain'' what she was doing on the flight and why she felt it necessary to stop in Athens where she spent seven hours in transit on her way to Beirut from Cairo. According to the same source, the person who sat in seat 10F where the explosion occured spent the whole flight with her table down. The source theorizes this hid her hands while she planted the bomb somewhere under the seat. The woman is alleged to have been Mansur.

Investigators are also asking how the terrorist got the explosives on board, escaping detection by security machines and TWA crew members.

According to a report from the Italian Interior Minister Oscar Luigi Scalfaro, the bomb may have consisted of a new type of plastic explosive that cannot be detected by the technology now used in airports. The damaged areas of the plane itself have been inspected, and autopsies are being performed by US and Greek forensic experts on the four victims of the blast, in part to look for traces of the explosive for analysis.

A TWA spokesperson said that although Mansur was late for the flight in Cairo and had to be driven by company car to the plane, she was put through rigorous security checks, including X-ray and human search of both hand-luggage and her person.

Last Thursday, Richard Petersen, the pilot of the aircraft, said he was entirely satisfied with the thoroughness of security checks made at all points on the Cairo-Athens-Rome leg of the journey. Some experts suggest that the recent reported use by TWA of less experienced crew members may have contributed to the failure to detect the explosive device.

If the terrorist did use a type of plastic explosive that cannot be detected by technology currently in use at airports, major changes in security measures at airports will have to be made, experts here say.

Greek officials are eager for the investigations to produce results. Harsh criticism of the government's attitude toward terrorism and of security conditions at the Athens airport, as well as several terrorist incidents last year, contributed to a sharp drop in tourism. After last week's incident, tour operators and tourism officials privately say they fear a decline of between 25 percent and 75 percent in arrivals from the US this summer.