Investigators won't be able to review the cockpit voice recording of a Qantas jet's mid-air crisis because it was overwritten as the crew made a harrowing emergency landing, an official said Tuesday.

The focus into what caused a hole in the 747-400's fuselage last Friday continues to narrow on a missing oxygen tank that appears to have burst while the plane was flying at 29,000 feet over the South China Sea last Friday, Neville Blyth, a senior investigator from the Australian Transport and Safety Bureau, told a news conference.

"The explanation regarding the loss of the cylinder is the most probable," Blyth said, adding that the 44-pound green tank was stored adjacent to the car-sized hole in the plane's metal skin and close to where a valve was found in the passenger cabin, likely blown through a hole in the floor.

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While tests still must be conducted on the valve and other fragments at an Australian laboratory, "it's safe to say that those components are from the missing cylinder," he said.

With air rushing out of the hole, the pilots made a rapid descent, then an emergency landing in Manila, the capital of the Philippines. Flight QF 30, with 345 passengers and a crew of 19, had been en route from London to Melbourne and had just made a stopover in Hong Kong.

The cockpit voice recorder and flight data recorder arrived Monday in Canberra, where officials discovered that the period when the crew was dealing with massive depressurization had been recorded over, Blyth said. The device operates on a two-hour loop.

"The oldest recording on the cockpit voice recorder commenced after the descent and diversion to Manila," Blyth said, adding it was still valuable in assessing the crew's handling of the situation.

The flight data recorder, which normally contains 25 hours of data, was being examined Tuesday.

A full search of the plane has yielded no trace of the missing cylinder, leading officials to conclude that it probably exited the plane through the fuselage.

Blyth showed reporters another tank that was stored next to the missing No. 4 cylinder. It was still in place and showed no signs of damage. The cylinders are used to provide oxygen to the passengers and crew during a high-altitude emergency.

Blyth and other officials say they are unaware of any previous cases in which an oxygen tank caused an airline accident. Qantas has ordered all oxygen tanks on its fleet of 747-400s to be inspected.

Qantas suffered another safety scare Monday when a landing gear door failed to close, prompting a flight to return to Adelaide shortly after takeoff.

Qantas insisted the plane was never in danger, but passengers already jittery over the Philippines incident described panic inside the Melbourne-bound plane after the pilot announced the problem.

"It was absolute chaos on the plane," passenger Gunter Kubler of South Africa told The Daily Telegraph. "They had to bring in another plane to fly people back, but I don't trust them, so I will take a bus or a train to Melbourne."

Airline spokeswoman Sophia Connolly insisted everyone on board the flight remained calm.

"In the scheme of things, this is pretty minor," Australian Civil Aviation Safety Authority spokesman Peter Gibson said. "It's unfortunate that things like this happen, but you've got to be realistic — planes are machines. From time to time, things go wrong."