The U.S. Federal Aviation Administration warned airlines to inspect oxygen cylinders on their planes months before the dramatic mid-air emergency last week aboard a Qantas flight that investigators suspect was caused by an exploding tank.

The air-worthiness directive, issued in April and effective in May, followed a report that certain oxygen cylinder supports in Boeing 747-400s may not have been properly heat-treated, which the FAA said could cause oxygen leakage and subsequent fire hazards.

Australian authorities say investigators are focusing on an oxygen bottle missing from the cargo hold of the Qantas 747-400 that was ripped open at 29,000 feet over the South China Sea on Friday.

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The plane, which was carrying more than 350 people, made an emergency landing in the Philippines.

Sydney-based Qantas Airways was ordered to urgently inspect every oxygen bottle aboard its fleet of 30 Boeing 747s, which is expected to take several days.

The FAA directive was originally issued in 2006 and reissued in April to include airplanes imported into the U.S. by American carriers.

It requires all airlines carrying Boeing 747-400 series aircraft to inspect the support brackets of the oxygen cylinders to determine their manufacturing date and perform any necessary corrective action.

"We are issuing this (directive) to prevent failure of the oxygen cylinder support under the most critical flight load conditions, which could cause the oxygen cylinder to come loose and leak oxygen," the FAA wrote. "Leakage of oxygen could result in oxygen being unavailable for the flight crew or could result in a fire hazard in the vicinity of the leakage."

Australia's Civil Aviation Safety Authority issued its own directive mirroring the FAA's in April.

A Qantas spokeswoman on Monday did not immediately respond to a request for comment. But an FAA spokesman warned that the cause of the Qantas emergency may be unrelated to the oxygen cylinder issues.

"That may or may not have anything to do with this incident," spokesman Les Dorr said Monday. "It's far too early to speculate on what the cause might be."

CASA spokesman Peter Gibson said an oxygen bottle explosion was one of several possibilities being investigated as a cause for the damage to the plane, and one that would be highly unusual.

"As far as we can determine this has never happened before on a passenger aircraft," Gibson told Australia Broadcasting Corp. radio.

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 (2.7-meter) hole in its fuselage, and the plane descended rapidly as debris flew through the cabin. The plane, en route from London to Melbourne, Australia, made a stopover in Hong Kong an hour earlier.

Four Australian Transport Safety Bureau specialists were inspecting the aircraft in Manila, with assistance from Boeing and the U.S. National Transportation Safety Board.

Officials have ruled out a bomb as the cause.