Air safety investigators are exploring whether interference from a nearby naval transmitter or even a passenger's laptop caused a Qantas jetliner to nose-dive twice over the Australian coast last month, an official said Friday.

Initial investigations of the mid-air emergency that left 13 seriously injured indicated the Oct. 7 malfunction was caused by a fault in a computer unit that uses sensors to detect the angle of the plane.

While that theory is still considered the most likely, investigators are looking into whether the fault could lie with electromagnetic interference from a low-frequency naval submarine communications transmitter on the Australian northwest coast at Exmouth, near where the plane made its emergency landing.

Another possible source of the interference is portable electronic devices, such as cell phones, Australian Transport Safety Bureau director Kerryn Macaulay said. She said even those considered safe to operate during flights, such as laptop computers, are being investigated.

"Possible external sources of electromagnetic interference are being explored and assessed," she said.

But even as investigators consider that theory, the plane's three computer units — called air data inertial reference units, or ADIRUs — will be examined next week at manufacturer Northrop Grumman Corp.'s factory in the United States, the bureau said in a preliminary investigation report.

The electromagnetic interference "is unlikely, especially if the problem is clearly identified during the ADIRU and the system testing," Macaulay said.

The plane was flying at 37,000 feet en route from Singapore to the western Australian city of Perth when a computer unit began transmitting wildly incorrect data to a main flight computer. The plane nose-dived 650 feet in 20 seconds before the crew brought it back to the original cruising level. The sharp drop was quickly followed by a second of about 400 feet in 16 seconds.

The aircrew made a mayday call to air traffic controllers after learning the extent of injuries on board, including broken bones, and detoured to the Learmonth air strip, near Exmouth. In all, 44 passengers and crew were injured.

The accident was one of a series of high-profile malfunctions and near-misses for Australia's flagship carrier in recent months.

The airline said another A330-300 on its way from Sydney to Shanghai turned back Thursday because of a weather radar malfunction on board.

Last month, the crew of a Qantas Boeing 747-400 was forced to rely on weather reports from an Air New Zealand airliner it was tailing across the Pacific Ocean after its own weather radar broke down.