An air safety investigator confirmed Wednesday that an exploding oxygen cylinder in the hold of a packed Qantas jetliner caused the large hole in the fuselage that forced the plane to make an emergency landing.

The explosion last Friday during a flight from London to Melbourne forced the pilots of the Boeing 747-400 to rapidly descend thousands of feet and make an emergency landing in the Philippines.

Australian Transport Safety Bureau director of aviation safety Julian Walsh said part of the oxygen tank blasted into the passenger cabin through the floor, smashed into a door handle and embedded in the ceiling.

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"The ATSB can confirm it appears that part of an oxygen cylinder and valve entered the passenger cabin" and hit a door handle, Walsh told reporters Wednesday.

"Clearly the valve has traveled vertically through the floor of the aircraft, glanced with the door handle and impacted with the ceiling of the cabin," he said.

But he said "there was never any danger of the door opening" because it is designed never to be opened while the plane is in the air.

Qantas was inspecting all its oxygen bottles aboard its fleet as a precaution, Walsh said.

"All of the evidence at the moment indicates the damage to the aircraft being caused by the failure of this bottle," Walsh said.

"This is a unique event. It's not happened before that we're aware of," he added.

The jet with 365 people aboard was flying at 29,000 feet when the explosion occurred in the cargo bay, rupturing the fuselage and causing rapid decompression in the cabin.

The pilots took about five and a half minutes to bring the jet down to 10,000 feet where oxygen was not needed. The ATSB is examining reports that fewer than 10 passengers were unable to get oxygen from their overhead masks during the steep decent.

Walsh said he did not know if Qantas would be able to repair the jet. Qantas has never lost an aircraft because of an accident.

"I don't really think that it's all that helpful to speculate on how close this is to a disaster," Walsh said.

"What we're dealing with here is the circumstances as they are. It's our job to look at these circumstances, to investigate it fully and to make sure that we find out what led to it so that we can make sure these things don't happen again," he added.

The ATSB, which is investigating the incident in cooperation with U.S. and Philippine authorities, will release a preliminary report in a month, Walsh said.