Malaysian investigators are trawling through the backgrounds of the pilots, crew and ground staff who worked on a missing aircraft for clues as to why someone on board flew it perhaps thousands of miles off course.

Background checks of passengers on Malaysian Airlines Flight MH370 have drawn a blank, but not every country whose nationals were on board has responded to requests for information, police chief Khalid Abu Bakar said.

No trace of the Boeing 777-200ER has been found since it vanished nine days ago with 239 people on board, but investigators believe it was diverted by someone who knew how to switch off its communications and tracking systems.

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Malaysia has appealed for more help in the search for the plane across two corridors stretching from the shores of Caspian Sea to the far south of the Indian Ocean.

"The search area has been significantly expanded," said acting Transport Minister Hishammuddin Hussein. "From focusing mainly on shallow seas, we are now looking at large tracts of land, crossing 11 countries, as well as deep and remote oceans."

The disappearance of the aircraft has baffled investigators, aviation experts and internet sleuths since it vanished from ­civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur en route to Beijing.

Malaysian authorities believe that as the plane crossed the country's north-east coast and flew across the Gulf of Thailand, someone on board shut off its communications systems and it turned sharply to the west.

Electronic signals it continued to exchange periodically with ­satellites suggest it could have continued flying for seven hours after flying out of range of ­Malaysian military radar off the country's north-west coast, heading towards India. The plane had enough fuel to fly for about seven-and-a-half to eight hours, Malaysian Airlines' chief executive Ahmad Jauhari Yahya said.

Police special branch officers have searched the homes of the captain, 53-year-old Zaharie Ahmad Shah, and first officer, 27-year-old Fariq Abdul Hamid, in middle-class suburbs of Kuala Lumpur close to the international airport.

An experienced pilot, Captain Zaharie has been described by current and former co-workers as a flying enthusiast who spent his days off operating a life-sized flight simulator he had set up at home.

Investigators had taken the flight simulator for examination by experts. Earlier, a senior police official said its programs were looked at closely, adding they appeared to be normal ones that allowed players to practice flying and landing in different conditions.

Police said they were looking at the backgrounds of both pilots and the other crew members. Ground support staff who might have worked on the plane were also being investigated.

No links between Captain Zaharie and any militant group had been found, though postings on his Facebook page suggest the pilot was a politically active opponent of the coalition that has ruled Malaysia for the 57 years since independence.

A day before the plane vanished, opposition leader Anwar Ibrahim was convicted of sodomy and sentenced to five years in prison, in a ruling his supporters and international human-rights groups say was politically influenced.