The nightmare scenario, now being presented by US investigators as the most plausible explanation for the disaster of EgyptAir Flight 990, has unnerved airlines, officials, passengers, and brought two governments to the brink of a confrontation. So far-reaching are the implications of the suicide dive theory that the government of Egypt on Tuesday officially contested the US conclusion, saying it was too premature, and sought a hold on Washington's move to hand over further investigation to the FBI.

Based on evidence from the cockpit voicerecorder and other flight data, here is what US investigators think might have happened: At about 1.49 am, when Flight 990 was cruising steadily at 33,000 feet, Captain al-Habashi left the cockpit, leaving the controls in the hands of a co-pilot who may have been al-Batouti.

Typical of long-distance international flights, there was at least one other pilot on board to enable rotation and beat fatigue. Although voice analysis suggests al-Batouti was co-piloting the plane at the time of the crash, he was not the designated co-pilot.

It is not clear where the designated co-pilot was. In fact, the plane had more than the usual complement of pilots, some as passengers.

Moments after al-Habashi leaves the cockpit, the co-pilot is heard muttering, ``Tawakilt ala Allah'', a common phrase that can roughly be translated as ``I put my faith in God,'' or ``I entrust myself to God.'' At the same time, the auto-pilot is disconnected and the plane begins to nose down. Al-Habashi returns to the cockpit and is heardsaying, in Arabic, ``What's going on?'' At this time, the plane goes into a steep dive taking the flight from 33,000 feet to 17,000 feet.

At 17,000 feet the plane levels off and begins to climb again getting to 24,000 feet. Investigators think there may have been a struggle in the cockpit because some of the instruments were being worked at cross purpose. Officials hypothesise that one pilot may have been desperately trying to pull out of the dive while the other pilot was forcing the plane's controls down. Near the bottom of the dive, someone shuts down the engines. Soon after the plane begins to climb again but it slows down so much that it cannot remain airborne and begins a deathly plunge again.

The US hypothesis has set off a frenzied speculation about suicide missions and terrorist infiltration. It has also convulsed aviation circles. Flight crews are routinely supposed to undergo medical and psychological evaluations, but how good are the tests? If the US theory is correct, then this would be thefirst instance in aviation history of a commercial airliner being brought down by a pilot on a suicide mission.

The hypothesis is so fraught with implications that Egypt, one of Washington's favourite allies, publicly sought to stall the transfer of the investigation to FBI, a move that would signal a criminal undertone to the disaster. The matter was considered serious enough for the Egyptian ambassador, Nabil Fahmy, to meet with US Under Secretary of State Thomas Pickering.

As it is, the Egyptian media and the bush telegraph in the Arab world is afire with various conspiracy theories about US and Israeli military involvement. One prominent Egyptian editor actually wrote a signed front-page editorial suggesting that US laser weapons may have brought down the plane, forcing the US ambassador in Cairo to protest. A theory that an Egyptian pilot may have deliberately brought down the plane is not going down well in Cairo.

While US investigators suggest the phrase, ``Tawakilt ala Allah'' is typicallyuttered by someone undertaking a significant action, Egyptian officials are believed to be arguing that it is a common expression that is uttered often in the most mundane of circumstances. Cairo is now sending its own Arabic experts to listen to the tone and inflection of the utterance.

Reports also say, the three pilots involved in the scenario had solid credentials. Al-Habashy, 57, had 26 years flying experience, and was a family man with children and grandchildren. The co-pilot Adel Anwar had been with Egypt Air for only five years, and was returning home to get married.

The focus of the investigation now, Al Batouti, was 59 and only a year away from retirement. Some reports said that Al-Batouti had told friends that this was his last flight before retirement while others suggested he had been depressed because he had not been promoted to captain.