Investigators who will try to determine the cause of the crash of EgyptAir Flight 990 already have some preliminary clues, but weeks or months of investigations will be required to determine how they fit among the elements that contributed to the tragedy.

So far, investigators know that the plane's electrical system apparently continued to work during at least the first part of the Boeing 767's plunge from the sky early yesterday because the plane's electrically powered radar transponder continued to report altitude data.

A rich database of radar tapes from multiple sites in the area will likely be able to help determine if and when the plane began to break up. If pieces began falling from the plane immediately, it would be likely that something violent happened to initiate the crash. However, pieces falling from a plane as it plunged straight down would be consistent with in-flight breakup caused by the forces building up during the dive.

Preliminary radar data from only one radar site indicate that the plane may have broken up shortly after its last transponder-reported altitude of 19,100 feet. A National Transportation Safety Board spokesman said the radar continued to pick up "primary hits"--those with no transponder data--for a few moments after the final transponder hit.

Further refinement will be required to determine when pieces began coming off the plane.

But a fuel tank explosion, such as the one that brought down TWA Flight 800 in 1996, is unlikely in this case. The TWA plane had been sitting on the ground for several hours on a hot July day, allowing fumes in the nearly empty center tank to become potentially explosive. Not only did the EgyptAir plane land on a cool evening, but it was on the ground much more briefly than the TWA plane. Sources also said the EgyptAir center tank had far more fuel than the TWA plane.

A sudden plunge from cruising altitude is rare. Most crashes occur on takeoff and landing, where evidence is easier to gather than from an ocean floor and where there may be witnesses. In the case of Flight 990, investigators must count on a combination of radar tracks, physical evidence, maintenance records and information from the plane's cockpit voice and flight data recorders, if they are recovered.

The 767 has an amazingly good safety record, with only three crashes, one of them during a hijacking. The crash of a Lauda Air 767 in a Thai jungle involved a rare mechanical problem, the in-flight deployment of a thrust reverser, in combination with the aerodynamic effect caused by the placement of the large engines on the wing. Normally, aviators have thought of in-flight thrust reverser deployment as a recoverable phenomenon. But investigators discovered the critical nature of the 767's wing-engine relationship using a supercomputer that was not available when the plane was made.

It is possible that safety board investigators will face some similar first-time mystery with Flight 990. As aviation becomes safer, each crash cause may be more complicated and bizarre than the last. For instance, a Russian Airbus crash was attributed to a pilot letting his son sit in the captain's seat. Unnoticed, the son disconnected the autopilot, according to the plane's recorders.

Among the possible causes the board must consider are pilot error, maintenance error, unusual weather phenomena or a bomb.

A bomb is one of the easiest to spot, assuming that wreckage is recovered from the area where it was placed. Explosives almost always leave unmistakable telltale marks. Investigators usually can tell exactly what kind of explosive was involved from sounds on a cockpit voice recording.

Also, U.S. intelligence agencies usually know whether the airline was under some credible threat and learn quickly from international informers if a terrorist act was involved. The fact that the FBI so quickly announced no evidence of a bomb indicates that there were no credible reports.

Unusual weather phenomena cannot be discounted. Planes are sometimes hit by violent turbulence that can kill unbelted passengers and damage an aircraft.

But human error usually is at the base of a crash, whether it is pilots who make a fatal mistake or react incorrectly to an emergency, or mechanics who install equipment the wrong way or fail to correctly conduct maintenance.

Pilots have been known to manipulate the wrong controls in an emergency or overstress the plane while conducting an emergency maneuver. The incapacitation of the pilots or even something as odd as an argument between them will have to be considered.

The list of possible errors would be almost endless. For instance, an American Airlines McDonnell Douglas MD-11 headed for South America underwent wild gyrations and injured passengers when a flight attendant placed a tray of soft drinks between the two pilots. Trying to be helpful and create room, the copilot unlatched the captain's chair, which startled the captain and caused him to jam into controls.

Investigators will look carefully at maintenance records, adherence to regulatory rules and orders and the competence and history of the crew, including medical records and their moods and activities during the last 72 hours of life.

CAPTION: A search vessel combs the crash site 60 miles off Nantucket, Mass.