Japanese airlines tonight began inspecting the tail sections of their Boeing 747 jumbo jets as experts investigating Monday's crash of a Japan Air Lines 747 pursued theories that fastening devices or rudders on the plane's tail somehow failed in mid-flight.

The inspections, to be performed in coming weeks on all sixty-nine 747s operated by Japanese carriers, were ordered this morning by the Japanese Ministry of Transport. The planes were not grounded in the meantime, however.

The Japan Air Lines jet crashed Monday evening with 524 passengers and crew after taking off from Tokyo on a domestic flight to Osaka. Four survivors were found Tuesday on the mountainside crash site 60 mile west of Tokyo, but all others aboard it are presumed dead in what appears to be the largest single-plane disaster in aviation history.

Airline oficials said that about 250 bodies had been recovered, and police said that 95 had been identified by relatives. The bodies were flown to a nearby town by helicopter.

Modern jet aircraft frames are constructed of high-strength metal. But vibration, shocks and general wear and tear can lead to formation of tiny cracks, which, if undetected, can cause components to snap without warning.

Theories that the accident began with structural failure in the tail began gaining credence after three large pieces from the stricken jet were found floating in the sea earlier this week about 80 miles south of the aircraft's impact point. A fourth piece, part of the tail, was found in the water today.

The first three have been identified as the upper part of the tail fin's leading edge, a lower section of the jet's rudder and an air vent that serves an auxiliary power unit mounted in the extreme rear of the plane's fuselage.

Loss of these parts would not have been visible from the cockpit. But they could explain why the plane appears to have flown aimlessly for about a half hour before crashing. In radio messages to the ground, the crew reported that they were "unable to control" the aircraft.

Investigators have also found major sections of the plane's tail fin at the crash site, indicating that it did not completely disintegrate in flight. Its tail wings have also been found among the wreckage.

The right rear door that the crew reported was "broken" has been found at the crash site hanging open on its hinges, with its lock mechanism intact. That eliminated an earlier theory that it had broken loose and damaged the tail section.

The recovery of pieces of the tail partly explains one of the radio transmissions from the pilot, staff writer Douglas Feaver reported from Washington. The leading edge of the vertical stabilizer -- the up-and-down part of the tail -- contains the antennas that would provide electronic navigational guidance to the cockpit. If the antennas were gone, important instruments would be rendered useless and the pilot would need navigational assistance. At one point, he asked air traffic controllers to pinpoint his location.

Although an aircraft is theoretically controllable even after the loss of a vertical stabilizer, the damage to the rear of the aircraft in addition could have caused a leak in the hydraulic systems that provide power assists to the flight controls. Without hydraulic assists, controls on the giant plane are nearly impossible to operate. The 747 has four independent systems, but a major leak could bleed off the hydraulic fluid after some period of time. The plane apparently flew for about 30 minutes after the initial damage.

Yumi Ochiai, an off-duty flight attendant who is among the four survivors, has said that the troubles began when a loud bang was heard above her seat, which was four rows from the back of the cabin.

Aviation experts have suggested that a pin, located almost directly above her seat and that fastens the front edge of the tail fin to the plane's fuselage, may have given way. That could have punctured the plane's body, causing the signs of rapid decompression that Ochiai reported.

Such explanations remain highly speculative, however. "Everything we can say at the moment is based on circumstantial evidence," a Transport Ministry technician involved in the investigation said today.

Investigators today began examining the jet's two flight recorders, which were recovered from the wreckage Wednesday, the Japanese news media reported.

The Ministry of Transport order today instructed airlines to inspect a series of bolts, rivets and metal tubes that fasten tail fins to the bodies of the aircraft. They were also told to inspect rudder hinges, balance weights on upper rudders and equipment that controls the rudders. They were told to check for leakage of hydraulic fluid from this equipment.

Inspection must be finished before 300 additional flight hours have elapsed for planes that have made fewer than 15,000 flights and within 100 hours for planes that have made more than 15,000 flights.

The job takes several hours. It entails visual examination and the painting of dye onto surfaces to detect invisible cracks. X-ray machines and cameras that can be inserted into the bodies may also be used.

Japan Air Lines planned to begin inspecting four of its jumbo jets tonight. All Nippon Airways, another major carrier, planned to inspect one plane.

In Washington, Feaver reported:

The Federal Aviation Administration said it had no present plans to order inspections of the 160 U.S.-operated 747s because not enough technical detail about the accident was yet available to make such inspections fruitful.

"There are no 747SRs, the version involved in the Japanese accident, being flown by U.S. airlines," the agency said in a statement. Spokesmen for the two major U.S. 747 operators -- Pan American World Airways and Northwest Orient Airlines -- added that they did not plan special inspections on their own.