CANBERRA, Australia — Investigators confirmed Friday that an exploding oxygen cylinder ripped a gaping hole in a Qantas jet's fuselage midflight last month, but said they were no closer to solving the mystery of why the tank failed.

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 told reporters.

The Boeing 747-438 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 "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. The cylinder's remains dropped through the ruptured fuselage and disappeared into the sea.

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.