Air safety investigators on Friday ruled out interference from a U.S. naval transmitter as the cause of a Qantas jetliner nose-diving twice off the Australian coast last year but failed to pinpoint what led the plane's computer to malfunction.

A second interim investigation report released Friday also failed to explain an explosion aboard another Qantas flight on July 25, 2008, that tore a gaping hole in the fuselage of a Boeing 747 over the South China Sea.

"Tests have revealed no evidence of an external explosive event or the use of explosive materials around the rupture area," AFP quotes the Australian Transport Safety Bureau (ATSB) report.

An earlier report said a passenger oxygen cylinder failed and exploded in the plane's hold, rupturing the fuselage, AFP reported. The explosion drove the cylinder through the cabin floor before it fell back into the hold and out of the aircraft.

That jet — on its way from London to Melbourne — made an emergency landing in the Philippines without injury to any of the 365 people aboard.

The ongoing Australian Transport Safety Bureau investigation into the Oct. 7, 2008, flight from Singapore to Perth — in which 12 passengers and crew were seriously injured — had examined whether the computer malfunction on the Airbus A330 was triggered by electromagnetic interference from a low-frequency U.S.-Australian naval submarine communications transmitter on Australia's northwest coast.

The A330 had been 100 miles from the transmitting station when it nose-dived 650 feet in 20 seconds before the crew brought it back to the original cruising altitude of 37,000 feet. The sharp drop was quickly followed by a second drop of about 400 feet in 16 seconds.

The aircrew detoured to an air strip near the station. In all, 44 passengers and crew were injured.

The Naval Communications Station, Harold E. Holt, was built by the U.S. Navy in the 1960s. It provides low frequency radio transmissions to the U.S. and Australian navies across the western Pacific and eastern Indian oceans.

Pilots have previously voiced suspicions that the transmissions interfered with aircraft instruments.

The report released Friday said the computer unit that caused the airliner to dive had been exposed under test conditions to an electromagnetic field 1,700 times stronger than it had experienced from the naval transmitter when it malfunctioned.

"None of the testing ... has produced any faults that were related to pitch-down events," the report said.

But the report found that the same unit — which feeds data about the plane's flying angle to the main flight computers — had malfunctioned two years earlier.

That malfunction did not upset the flight and no fault could be identified.

In December, the same type of unit malfunctioned on another A330 flying from Perth to Singapore.

The air crew followed new procedures developed in response to the Oct. 7 emergency by switching off the unit and returned to Perth without mishap. That fault remains under investigation.

The investigation into the 747 emergency reported in August that an oxygen bottle had exploded below the passenger cabin floor, damaging flight instruments and rapidly decompressing the cabin.

The interim report on Friday said similar bottles had been tested but no design faults could be found.

Qantas welcomed the reports as supporting its own conclusions that the airline had not been at fault.

"These two incidents involved extremely rare, if not unique, circumstances that were beyond Qantas' control," airline chief executive Alan Joyce said in a statement.

The Associated Press contributed to this report.