Investigators probing wreckage from China Airlines Flight 611, which flew apart at 35,000 feet over the Taiwan Strait, have discovered a series of fatigue cracks in the rear fuselage area near a 22-year-old repair, sources close to the investigation said today.

The Boeing 747-200 suddenly broke up May 25 about 20 minutes after taking off from Taipei for Hong Kong, killing all 225 people aboard. There was no distress call from the crew, and so far the plane's cockpit voice recorder and flight data recorder have revealed no definitive cause for the rare high-altitude disintegration.

The fatigue cracks are the first physical evidence pointing to a possible cause, and investigative sources said that area of the fuselage "is getting a lot of attention." But the sources stressed that much work remains to be done before Taiwan's Aviation Safety Council can determine whether the fatigue cracks played a role in the crash or some other initiating event merely caused the fuselage to crack apart at that point.

The cracks raise two questions for investigative and regulatory agencies from Taiwan and the United States: First, were they the initiating event in the crash? And second, are the cracks a one-time defect caused by a repair, or should all older 747's be inspected for hidden cracks?

So far, several sources said, the cracks appear to be a one-time event, but not enough wreckage has been recovered from the ocean floor yet to make a final determination.

"We don't see a need to act yet," said one official.

The in-flight breakup does not appear to be the result of an explosion or fire. Officials have said they do not see any telltale burns or blast damage. Nor does it appear to be a fuel tank explosion, like the one that brought down Trans World Airlines Flight 800 in 1996.

A fatigue crack grows over a period of time in metal, sometimes because of damage or flexing.

The cracks found in the rear fuselage of the China Airlines plane, at least one of which was 40 inches long, were all in the vicinity of a two-foot-by-10-foot "doubler," essentially a metal patch used to repair damage caused in 1980 by a "tail strike." In a tail strike, the fuselage rubs along the runway, usually when an airplane takes off at too high an angle.

The Taiwanese investigative agency is only now getting maintenance paperwork that will tell who performed the repairs and exactly what was done.

So far, salvage operations have located only one side of the fuselage at the fatigue crack that gave way. Investigators are anxious to locate the mating piece.

The aircraft has been located on the ocean floor in roughly three large debris fields. The first debris field consists mostly of the tail section up to the area of the tail strike repair. The second debris field, about a mile away along the flight path, appears to contain most of the rest of the plane.

The third debris field appears to contain two engines. Sources said it appears that the engines were not the cause of the crash, but cracked off as the plane broke apart.

Kay Yong, managing director of Taiwan's Aviation Safety Council, said earlier that the cockpit voice recorder contains a sound at the end of the recording that technicians have been unable to identify, and that the flight data recorder shows only small anomalies in the last 20 seconds.

Among other things, the 747 appears to pitch up somewhat abruptly five seconds before the end of the recording. The cockpit voice recorder, which contains the last 30 minutes of cockpit sounds and conversation, continued to operate for three seconds after the flight data recorder cut off.

Yong said that in the last second before the voice recorder cut off, there was "a not very loud 'chahhh' sound" that has not been identified. There are also a few other unidentified sounds during the last seven minutes of the recording that specialists find unusual for a 747, but they have been unable to identify them.

Fatigue cracks in older aircraft are not a new issue. The Federal Aviation Administration initiated an "aging aircraft" inspection program after part of the roof tore off an older Aloha Airlines Boeing 737 in April 1988. The National Transportation Safety Board found that Aloha had failed to inspect the aircraft adequately for cracks.

The 747 also has experienced trouble with cracks in the area of its forward door, but not at the rear of the aircraft. The cracking around the forward door has been handled with regular inspections and repairs to tiny cracks before they can grow bigger.