Investigators are now convinced the missing Malaysia Airlines plane was hijacked by one or more people with significant flying experience, who switched off communications and diverted the flight, an official involved in the investigation said on Saturday.

But they do not know the motive or where the plane was taken, the unnamed source told Associated Press.

“It is conclusive,” said the Malaysian official, who spoke anonymously because he is not authorised to brief media.

The huge multinational search was focused on the Bay of Bengal early on Saturday, one week after flight MH370 vanished, as US officials confirmed they had directed surveillance aircraft to patrol the area for debris.

There were reports that Malaysian military radar indicated the plane made at least two distinct changes of course after apparently turning back from its route towards Beijing. US officials indicated that they believed the plane had crashed in the Indian Ocean and said that an aerial search of the area would begin on Saturday.

The Malaysian official said it had been established with a “more than 50 percent” degree of certainty that military radar had picked up the missing plane after it dropped off civilian radar.

But a new report has claimed the Malaysian Airlines plane could have been flown off the coast of Australia - still over the Indian Ocean, but thousands of kilometres south of the focus of the search.

Bloomberg cited a person familiar with the analysis, who said the last contact with a satellite showed MH370 around 1,000 miles west of Perth, but added that might not indicate where the plane ended up.

If the missing airliner crashed in the Indian Ocean, which plunges to depths of 7,000m (23,000ft), it would mean a significant escalation in scale of the challenge facing investigators. Any debris could have been swept far from the original crash site.

The last communication with the crew was made at around 1.20am, 40 minutes into the flight, as it headed east over the South China Sea towards Vietnam. The plane had enough fuel to fly for another five hours – meaning its potential range was enormous.

Investigators believe that one or more people switched off communications devices and steered the plane off course, according to the AP source.

Both military radar readings and the plane’s automatic attempts to establish contact with satellites have offered key clues to its whereabouts, suggesting it flew for four to five hours and was last seen heading north-west towards the Andaman Islands.

Experts say that while changes in altitude could be caused by fuel burning off, they would not account for the changes in direction. The New York Times also reported that the changes appear to have taken the plane both above and below usual cruising levels for a Boeing-777 at various points in its journey, with it climbing to 45,000 feet before turning west and descending to 23,000 feet as it approached Penang.

Earlier, an American official told AP that investigators are examining the possibility of “human intervention” in the plane’s disappearance, adding it may have been “an act of piracy.”

The official suggested a key piece of evidence suggesting intentional interference with communications was that that contact with the Boeing 777’s transponder stopped about a dozen minutes before a messaging system on the jet quit - making it less likely a sudden catastrophic failure was to blame.

Some experts have said sequential failures due to technical problems were not impossible - for example if there was a fire - though they would be unusual.

It also appeared to be steered to avoid radar detection.

Malaysian police said earlier this week they would be investigating the backgrounds of two pilots, ten crew members and all 227 passengers.

The Wall Street Journal reported that manually dismantling communications systems – such as the transponder, which communicates the aircraft’s position, speed and call sign to air traffic control radar – would have required detailed knowledge of the workings of the Boeing-777.

It said investigators are also trying to determine why the plane stopped pinging satellites after five hours while apparently cruising over the Indian Ocean. That could be caused by disconnecting the system – an extremely complex task – or by something catastrophic happening to the flight, an expert told them.

The Malaysia Airlines flight was bound for Beijing when it vanished. Numerous nationalities were among the 239 on board but about two-thirds were Chinese.

A commentary carried by China’s state news agency Xinhua demanded: “Why is the silence on the flight being kept so long?”

Complaining that officials had been largely silent, it added: “Mounting evidence points to the theory that, including the possibilities of pilot error or terrorist activity, the loss of MH370 with 239 people on board is a man-made event rather than the result of a mechanical breakdown.

“If sabotage is not ruled out, withholding information from the public can be dangerous, even lethal.”