MANILA (Reuters) - A cockpit voice recorder gave no useful information about what had torn a large hole in the fuselage of a Qantas Airways 747 plane last week, forcing it to make an emergency landing, investigators said on Tuesday.

Neville Blyth, senior investigator from the Australian Transport Safety Bureau (ATSB), said the recorder used a continuous loop and could only record the last two hours of conversation in the cockpit.

It did not capture what caused a hole the size of a mini-van to be ripped into the right side of the aircraft's fuselage, resulting in a sudden loss of cabin pressure which forced the flight crew to make an emergency landing in Manila.

Blyth said investigators were forced to look for other recorded data on the aircraft, as well as maintenance records of about a dozen oxygen cylinders, to get a picture of what happened during the Hong Kong to Melbourne flight.

One theory is that an exploding oxygen cylinder ruptured the fuselage. One of the cylinders aboard the plane is missing and some of its components were found near the hole.

"The maintenance is going to be the sharp focus," Blyth said, showing one of the 20 kg green-colored cylinders.

"We're looking at the manufacture of the cylinder, the condition of the cylinder and any aspect of the installation of the cylinder," he said. "This is not a simple matter. There's a lot of ... information that needs to be gathered before any conclusions can be drawn."

The ATSB has appealed to passengers to provide information, including pictures and videos taken aboard the plane. All 346 passengers and 19 crew disembarked safely in Manila.

(Reporting by Manny Mogato; Editing by Raju Gopalakrishnan and Paul Tait)

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