CANBERRA, Australia — An oxygen tank exploded and blew a car-sized hole in a Qantas jet last month, air safety officials said today, but investigators appear to be no closer to figuring out why.

The release of the interim report by Julian Walsh, acting executive director of the Australian Transport Safety Bureau, confirmed earlier suspicions by investigators that the tank was the cause.

“We don’t really know why the bottle failed — and that’s the key question for the investigation,” Walsh said in releasing the report. He said the investigation will likely continue for months.

The Boeing 747-400 aircraft, carrying 365 people, was flying over the South China Sea July 25 when the explosion blew a hole in the fuselage 79 inches wide and 60 inches high, the report said.

Walsh said one of the seven emergency oxygen cylinders below the cabin floor had exploded.

The 26-pound steel cylinder, pressurized to 1,850 pounds per square inch, “sustained a failure that allowed a sudden and complete release of the pressurized contents,” Walsh said.

Most of the cylinder rocketed up through the cabin floor, shearing off an emergency exit door handle and narrowly missing a crew seat before striking the cabin roof. It ricocheted back down through the hole it created in the cabin floor, the report said.

Walsh said the cylinder had undergone a safety inspection shortly before it was installed in the jet and six weeks before it exploded.

The plane — en route from London to Melbourne, Australia — rapidly descended thousands of feet with the loss of cabin pressure and flew about 300 miles to Manila, where it made a successful emergency landing.

No one was injured, but questions were raised about the much-lauded safety of Qantas Airways, which has never lost a jet aircraft because of an accident.

In the weeks after the incident, Qantas planes experienced a number of other problems, including a loss of hydraulic fuel that led to an emergency landing, failure of landing gear, and detached panels.

The problems prompted the Civil Aviation Safety Authority, Australia’s aviation agency, to launch a review of Qantas Airways’ safety standards.

Qantas Airways backed the bureau’s findings.

“The preliminary report was a factual account of the incident and investigation to date,” Qantas chief executive Geoff Dixon said in the statement. “Our own investigations agree with the ATSB’s preliminary conclusions.”

Qantas earlier this month temporarily pulled six planes from service because of irregularities in maintenance records. Qantas said it was a record-keeping issue and there were no safety implications for the aircraft.