The Boeing 747-438 aircraft, carrying 365 people, was flying over the South China Sea July 25 when the explosion blew a hole in the fuselage five-feet in diameter, causing a loss of cabin pressure.
Walsh said one of the seven emergency oxygen cylinders below the cabin floor had exploded.
"On the basis of the physical damage to the aircraft's forward cargo hold and cabin, it is evident that the number 4 passenger oxygen cylinder sustained a failure that allowed a sudden and complete release of the pressurized contents," Walsh told reporters in releasing the report.
The plane -- en route from London to Melbourne, Australia -- rapidly descended thousands of feet and flew about 300 miles to Manila, where it made a successful emergency landing.
No one was injured, but questions were raised about the much-lauded safety of Qantas Airways, which has never lost a jet aircraft because of an accident.
In the weeks after the incident, Qantas planes experienced a number of other problems, including a loss of hydraulic fuel that led to an emergency landing, failure of landing gear, and detached panels.
The problems prompted the Civil Aviation Safety Authority, Australia's aviation agency, to launch a review of Qantas Airways' safety standards.
Qantas Airways backed the bureau's findings.
"The preliminary report was a factual account of the incident and investigation to date," Qantas chief executive Geoff Dixon said in the statement. "Our own investigations agree with the ATSB's preliminary conclusions."
Qantas earlier this month temporarily pulled six planes from service because of irregularities in maintenance records. Qantas said it was a record-keeping issue and there were no safety implications for the aircraft.
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