Australian aviation officials said Friday a computer glitch made a Qantas jet dive in mid-air and they were investigating if it was due to a military transmitter or passengers` electronic devices.

The Airbus A330 from Singapore to Perth was cruising at 37,000 feet west of the Australian town of Learmonth on October 7 when it twice abruptly plunged.

It left 13 passengers and a flight attendant with serious injuries and many of the other 313 onboard with more minor ailments from being flung around the cabin.

In a preliminary report released Friday, the Australian Transport Safety Bureau said its inquiry indicated that a fault in a computer system component had sent spurious data to flight control, causing the autopilot to shut down.

But what caused the fault in the first place is unclear.

ATSB spokeswoman Kerryn Macaulay said the bureau was investigating why the spikes in data occurred and why they were sent to the flight control equipment unchecked.

She said comprehensive testing of the usually highly reliable component -- known as the Air Data Inertial Reference Unit (ADIRU) -- would begin Monday in the United States.

The report said other possible factors would also be studied, including an Australian naval communications transmitter near Exmouth, near Learmonth, and portable electronic devices on board the aircraft.

`This is unlikely, especially if the problem is clearly identified during the ADIRU and system testing,` the ATSB said in a statement.

Macaulay said it was too early to speculate what caused the computer system to malfunction, but that the bureau could not rule out the possibility it was related to the transmitter.

`Until we do the testing on the ADIRU and the other flight systems we won`t rule it out completely as a possibility,` she told reporters in Canberra.

Macaulay said investigators were still seeking information from passengers about the use of laptop computers and mobile phones during the flight.

`Where necessary we will look into that issue more significantly, particularly if we are able to rule out software and hardware problems with the system onboard the aircraft itself.`

The aircraft first plunged 650 feet (200 metres) but the crew were able to return it to altitude. However, it then dived a second time, descending around 400 feet, before the pilots regained control and made an emergency landing at Learmonth.