Investigators probing Monday's crash of a Japan Air Lines jumbo jet are studying the possibility that a pressure seal at the back of the cabin burst, releasing compressed air that then tore away parts of the tail.

The rear bulkhead, a partition that seals the pressurized cabin from the nonpressurized tail section, was found badly damaged today at the crash site. Attention turned to a theory that the bulkhead gave way and released explosive air pressure from inside the cabin that filled the hollow vertical stabilizer, causing it to explode.

Air could rush from the cabin to the tail section at up to 742 mph, or about the speed of sound, Prof. Hiroshi Maeda of Kyoto University, a specialist in aircraft structure, told The Associated Press.

Hiroshi Fujiwara, an official of the government committee investigating the crash, said the bulkhead was "peeled like an orange." He said studies will be conducted to see if it failed due to metal fatigue or was broken by some outside force.

At least 30 pieces from the plane's rear section have now been picked up in waters south of Tokyo. Investigators are trying to confirm how much of the tail was still in place when the jet went down Monday night with the loss of 520 lives.

Japanese television reported tonight that one of the pieces in the water was a section of the passenger cabin ceiling, but a spokesman for Japan Air Lines said he was unable to confirm it.

News reports Wednesday quoted Yumi Ochiai, an off-duty flight attendant who survived the crash, as saying that she had seen "blue sky" through a hole in the cabin ceiling before the crash, but an airline spokesman said at the time that he was unable to confirm the statement. She was seated in the tail section of the plane.

In a statement released by the airline, Ochiai described a series of occurences before the crash that suggested loss of cabin pressure. She said she heard a loud explosion, after which the air in the cabin turned hazy and oxygen masks dropped from the ceiling for passengers.

Nine U.S. air crash experts were flown by helicopter to the crash site today to join the investigation.

By late today, crews had recovered 392 bodies from the wreckage, the airline reported. Of these, 202 have been identified and many families have begun conducting cremation funerals.

Only four of the 524 people on the Boeing 747 survived the crash, making it the worst single-plane disaster in aviation history.

Tail sections are crucial to control of an aircraft. The jet's tail appears to have broken apart about 15 minutes after takeoff from Tokyo's Haneda Airport shortly after 6 p.m. Monday. The plane flew out of control for about 40 minutes before crashing on the mountainside 60 miles west of Tokyo.

Aviation experts have theorized that metal fatigue in components that fasten the tail fin, or vertical stabilizer, to the fuselage may have caused them to break suddenly.

They are also looking at the possibility of a failed bulkhead. The so-called "explosive decompression," the rapid escape of air from the pressurized cabin that this could have unleashed, has figured in a number of aviation accidents in the past, most notably the crash of a Turkish Airlines DC10 in 1974 that killed 346 people.

JAL technical manager Hiroaki Kohno told reporters the bulkhead on the crashed plane was damaged when the plane's tail scraped a runway on landing at Osaka in 1978. The lower portion was replaced by Boeing engineers as part of overall repairs, he said.

But some aviation specialists here believe the plane may have sustained damage that was not detected. The tail hit the runway with such force that more than 20 passengers were injured.

In Seattle, the Boeing Co., which made the plane, issued a "service advisory" to all 747 operators, saying they may wish to inspect sections of the planes near the tail.

On Thursday, the Japanese Ministry of Transport ordered Japanese airlines to inspect the tail sections of the 69 Boeing 747s they operate. Japan Air Lines reported that it completed inspection on four last night and found nothing wrong.

Japan's largest daily newspaper, the Yomiuri Shimbun, published a grainy photograph taken by an amateur photographer, showing a silhouetted jet flying overhead with most of its tail fin missing.

The photographer, Keiichi Yamazaki, said in a telephone interview he was in his garden in Okutama, 40 miles west of Tokyo, at 6:50 p.m. Monday when he noticed the plane "flying very low, with a lot of noise."

It was unusual, he said, because aircraft rarely fly over his area. "I grabbed my camera and took one shot. Then the plane disappeared."