Investigators said yesterday that an "abnormal impact" in the tail section of a Japan Air Lines Boeing 747 apparently set off events that led to the Aug. 12 crash of the plane and the deaths of 520 persons.

What caused the impact is undetermined, according to wire service accounts of a 60-page interim report on the world's worst single-plane accident.

U.S. sources said that the damage to the aircraft's tail section is consistent with what could happen if the aft pressure bulkhead -- the rear wall of the passenger cabin -- exploded outward as the plane climbed to high altitude and its cabin was pressurized for passenger comfort.

"The indications are that a bulkhead failure would have resulted in the kind of damage that took place," one source said. "There is also no question there was an explosive decompression."

There were four survivors. They have described a mist filling the cabin as the oxygen masks deployed automatically in the suddenly rarefied air. The plane continued to fly for almost 30 minutes after a loud bang was registered on the plane's cockpit voice recorder 12 minutes after takeoff.

After the bang, the pilot was unable to control the aircraft. A photograph of the plane in flight shows that a large portion of the vertical tail section was missing. The pilot noted that his hydraulic systems had failed, and they provide essential "power steering" for 747 controls.

Investigators have not ruled out a bomb or a midair collision with an unspecified object as a possible cause of the crash. They also think the bulkhead could have collapsed as a result of some other event not yet understood.

According to Reuter, JAL's managing director, Satori Totoki, told the Asahi newspaper that "the bulkhead of that plane was inspected scrupulously last December . . . . If there were cracks they would have been discovered. It is not possible that there were cracks in the bulkhead linked to the breakup of the tail fin."

Repairs made after a 1978 incident involving the same plane are high on the suspect list. In that incident, the plane's tail struck the ground hard on landing and damaged the bulkhead, which was repaired with Boeing's help.

The JAL crash came almost two months after an Air India 747 disintegrated off the Irish coast and killed all 329 persons aboard. International aviation safety authorities sent a special photographic mission into the Irish Sea to inspect the Air India wreckage, and the cameras found the tail section intact.

It is widely assumed that a terrorist bomb caused that crash, but investigators have yet to find any physical proof. That investigation also is continuing.