Qantas airlines rushed to inspect oxygen cylinders on its entire fleet Monday as investigators focused on a missing tank as the suspected cause of a mid-air blast that tore a hole in a jumbo jet carrying more than 350 people, forcing an emergency landing in the Philippines.

Australia's Civil Aviation Safety Authority said the Sydney-based airline was ordered to quickly inspect every oxygen bottle aboard its 30 Boeing 747s.

Civil authority spokesman Peter Gibson confirmed an oxygen cylinder was missing from the Boeing 747-400 that landed in Manila on Friday after a section of its metal skin was ripped away at 29,000 feet over the South China Sea. There were no injuries.

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"If it turns out that is the cause of the accident, the cause of the hole in the side of the aircraft, obviously that will be a key part of the investigation working out why a bottle would suddenly give way," Gibson told Australian Broadcasting Corp. radio on Monday.

"As far as we can determine this has never happened before on a passenger aircraft," he said, adding the possibility was "very unusual and obviously understanding why that happened will be absolutely critical to making sure it can't occur again."

He said a possible cause of the blast could include metal fatigue in the cylinder, a failure of the regulator valve, something hitting it and puncturing it, or the cylinder becoming too hot.

A senior investigator from the Australian Transport and Safety Bureau, Neville Blyth, told reporters Sunday the incident was treated as a safety investigation.

"At this stage, there is no evidence whatsoever that this is a security-related event," he said. An official of the U.S. Transportation Security Administration also said initial reports indicated no link to terrorism.

Gibson said an inspection of all oxygen canisters in Qantas' fleet will take several days. He said tanks located near the hole contained emergency oxygen for the flight deck.

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 formed.

Verdier said it was too soon to say what caused the explosion or whether the canisters may have contributed to the blast.

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.

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.