Australia's Civil Aviation Safety Authority said Qantas has been ordered to urgently inspect every oxygen bottle aboard its fleet of 30 Boeing 747s.
"At this stage, there is no evidence whatsoever that this is a security-related event," Neville Blyth, senior investigator from the Australian Transport and Safety Bureau, told a news conference in Manila. "This is being treated as a safety investigation."
Blyth said tests for bomb residue were negative. Philippine bomb-sniffing dogs went through the aircraft, particularly the cargo hold and the passenger baggage, and found no indication of explosives.
He said the focus is now on an oxygen bottle missing from the cargo hold that was left exposed when a section of the 747-400's metal skin ripped away at 29,000 feet over the South China Sea on Friday.
"I can't speculate as to indeed the probability of that cylinder having caused the damage," Blyth said, when asked if there were indications that the scuba tank-like cylinder had exploded and damaged the plane.
"In the vicinity of the damage, we are missing one cylinder. The areas around the damage will be inspected. We're obviously looking for evidence on where that cylinder may have gone," he said.
Peter Gibson, spokesman for the Civil Aviation Safety Authority, said an inspection of all oxygen bottles in Qantas' fleet will take several days. He said bottles located near the hole contained emergency oxygen for the flight deck.
Passengers described the plane being shaken by a loud bang. Oxygen masks dropped from the ceiling due to rapid decompression caused by the 9-foot hole in its fuselage, and the plane descended rapidly as debris flew through the cabin. The plane, en route from London to Melbourne, Australia, had made a stopover in Hong Kong an hour earlier.
Four Australian Transport Safety Bureau specialists began inspecting the aircraft Saturday and were expected to continue their work for two or three days with assistance from Boeing and the U.S. National Transportation Safety Board, Blyth said.
The plane's flight data recorder, cockpit voice recorder and a small disc that records maintenance details have been sent to Australia to be analyzed "in respect of the handling of the aircraft," Blyth said.
The possibility of an explosion is one of several scenarios being considered by investigators, said Julian Walsh of the Australian Transport Safety Bureau.
"There are oxygen cylinders contained in the cargo compartment," he told reporters. "The relevance of that will certainly be covered in the investigation."
An official of the U.S. Transportation Security Administration said initial reports indicated no link to terrorism.
Some passengers told Australian media that their oxygen masks failed to work properly during the crisis, causing some to nearly pass out.
Other passengers, while applauding the pilot and crew's performance, told of having to share oxygen masks among three people.
"Ours didn't come down, and my husband just about (passed out) because he didn't have any oxygen for about three minutes," Beverley Doors told Australian Broadcasting Corp. radio.
Passenger David Saunders said one man in front of him smashed the ceiling panel to force his mask to come down and that children were screaming and flailing.
"Their cheeks and lips were turning blue from lack of oxygen," he said.
Boeing spokeswoman Liz Verdier said the design of the Qantas jet includes dozens of oxygen tanks located throughout the lower part of the aircraft, including below the passenger compartment where the hole is.
Gibson of Australia's Civil Aviation Safety Authority said speculation that corrosion contributed to the accident could be discounted.
"It's clearly an extremely rare and unusual event that a hole opens up in the fuselage," he told reporters in Australia. "I know there's a number of theories around, but they're just that at this stage, they're just theories. We don't have the solid facts."
Blyth, however, said investigators will check whether there was any corrosion in the aircraft wall or the oxygen cylinders that may have caused a problem.
Qantas boasts a strong safety record and has never lost a jet to an accident. Its last crash of a smaller plane was in 1951.