There is no evidence that explosive materials were involved in a mid-air blast that forced a Qantas airliner to make an emergency landing last year, Australian air safety investigators said on Friday.
An exploding oxygen bottle has already been blamed for blowing a gaping hole in the Boeing 747-400 carrying 365 passengers from Hong Kong to Australia in July, but the latest findings take the probe further.
"Tests have revealed no evidence of an external explosive event or the use of explosive materials around the rupture area," the Australian Transport Safety Bureau (ATSB) said in an interim report.
A preliminary ATSB report released in August said one of seven passenger oxygen cylinders failed and then exploded in the aircraft hold, rupturing the fuselage.
The explosion punched through the cabin floor, driving the malfunctioning cylinder into the cabin before it fell back through the floor and out of the aircraft, it said.
Terrorism was ruled out soon after the incident, but the latest report said investigators had been unable to find a reason for the cylinder explosion, with the only item of physical evidence recovered being the oxygen valve.
"The number four cylinder was not recovered and is presumably lying on the bottom of the South China Sea," the report said.
The valve showed no sign that "an oxygen-promoted fire or an overpressure event had contributed to the cylinder failure," it said.
A final report on the incident, in which nobody was injured, is expected towards the end of the year.