* Total search area totals 7.68 million sq km - roughly size of Australia
* Background checks find no clear motive for plane's disappearance
* Thailand says did detect plane re-crossing peninsula
* Malaysia requests radar, satellite data to help narrow search
* Police study runways programmed into pilot's flight simulator
By Anshuman Daga and Niluksi Koswanage
KUALA LUMPUR, March 18 (Reuters) - An international land and sea search for a missing Malaysian jetliner is covering an area the size of Australia, authorities said on Tuesday, but police and intelligence agencies have yet to establish a clear motive to explain its disappearance.
Investigators are convinced that someone with deep knowledge of the Boeing 777-200ER and commercial navigation diverted Malaysia Airlines Flight MH370, carrying 12 crew and 227 mainly Chinese passengers, perhaps thousands of miles off its scheduled course from Kuala Lumpur to Beijing.
But intensive background checks of everyone aboard have so far failed to find anyone with a known political or criminal motive to hijack or deliberately crash the plane, Western security sources and Chinese authorities said.
Malaysian Acting Transport Minister Hishammuddin Hussein told a news conference the "unique, unprecedented" search covered a total area of 2.24 million nautical miles (7.68 million sq km), from central Asia to the southern Indian Ocean.
Flight MH370 vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after take-off early on March 8.
Investigators piecing together patchy data from military radar and satellites believe that someone turned off the aircraft's identifying transponder and ACARS system, which transmits maintenance data, and turned west, re-crossing the Malay Peninsula and following a commercial aviation route towards India.
Malaysian officials have backtracked on the exact sequence of events - they are now unsure whether the ACARS system was shut down before or after the last radio message was heard from the cockpit - but said that did not make a material difference.
"This does not change our belief, as stated, that up until the point at which it left military primary radar coverage, the aircraft's movements were consistent with deliberate action by someone on the plane," said Hishammuddin. "That remains the position of the investigating team."
BACKGROUND CHECKS CLEAN
China's ambassador to Malaysia said his country had carried out a detailed probe into its nationals aboard the flight and could rule out their involvement.
U.S. and European security sources said efforts by various governments to investigate the backgrounds of everyone on the flight had not, as of Monday, turned up links to militant groups or anything else that could explain the jet's disappearance.
A European diplomat in Kuala Lumpur also said trawls through the passenger manifest had come up blank.
One source familiar with U.S. inquiries said the pilots were being studied because of the technical knowledge needed to disable the aircraft's communications systems.
The New York Times cited senior U.S. officials as saying that the first turn back to the west was likely programmed into the aircraft's flight computer, rather than being executed manually, by someone knowledgeable about aircraft systems.
Malaysia Airlines Chief Executive Ahmad Jauhari Yahya told Tuesday's daily news conference that that was "speculation", adding: "Once you are in the aircraft, anything is possible."
Malaysian officials said on Monday that suicide by the pilot or co-pilot was a line of inquiry, although they stressed that it was only one of the possibilities under investigation.
Malaysian police have searched the homes of the captain, Zaharie Ahmad Shah, 53, and first officer, Fariq Abdul Hamid, 27, both in middle-class suburbs of Kuala Lumpur close to the airport.
Among the items taken for examination was a flight simulator Zaharie had built in his home.
A senior police officer with direct knowledge of the investigation said the programs from the pilot's simulator included Indian Ocean runways in the Maldives, Sri Lanka, Diego Garcia and southern India, although he added that U.S. and European runways also featured.
"Generally these flight simulators show hundreds or even thousands of runways," the officer said.
"What we are trying to see is what were the runways that were frequently used. We also need to see what routes the pilot had been assigned to before. This will take time, so people cannot jump the gun just yet."
"NEEDLE IN A HAYSTACK"
Thailand said on Tuesday a re-examination of its military radar data had picked up the plane re-tracing its route across Peninsular Malaysia. The Thai military had previously said it had not detected any sign of the plane.
What happened next is less certain. The plane may have flown for another six hours or more after dropping off Malaysian military radar about 200 miles northwest of Penang Island.
But the satellite signals that provide the only clues were not intended to work as locators. The best they can do is place the plane in one of two broad arcs - one stretching from Laos up to the Caspian, the other from west of Indonesia down to the Indian Ocean off Australia - when the last signal was picked up.
Malaysia has asked countries along both corridors, as well as others with satellite capabilities, to re-examine their data to try to narrow the search area, Hishammuddin said.
China, which, with Kazakhstan, is leading the search in the northern corridor, said on Tuesday it had deployed 21 satellites to scour its territory.
"In accordance with Malaysia's request, we are mobilising satellites and radar to search over the Chinese section of the northern corridor, which the Malaysians say the plane may have flown over," said Foreign Ministry spokesman Hong Lei.
Australia, which is leading the southernmost leg of the search, said it had shrunk its search field based on satellite tracking data and analysis of weather and currents, but that it still covered 600,000 sq km (230,000 sq miles).
"A needle in a haystack remains a good analogy," John Young, general manager of the emergency response division of the Australian Maritime Safety Authority (AMSA), told reporters.
"The aircraft could have gone north or south, and if it went south, this is AMSA's best estimate of where we should look with the few resources we have at our disposal for such a search."
The U.S. Navy was sending a P-8A Poseidon, its most advanced maritime surveillance aircraft, to Perth, in Western Australia, to assist with the search.
The disappearance of the plane was a major topic of conversation at the International Society of Transport Air Trading in San Diego, an annual gathering of 1,600 airplane makers, buyers and lessors.
"The people that I deal with are looking at this with great concern - it appears considerable efforts may have gone into cloaking the aircraft," said Robert Agnew, chief executive of aviation consultants Morten Beyer & Agnew.