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A Royal Australian Air Force pilot of an AP-3C Orion maritime patrol aircraft scans the surface of the sea near the west of Peninsula Malaysia in this handout picture by the Royal Australian Air Force, released via the Australian government's Department of Defence website on March 17, 2014. [Photo/Agencies]

Passengers' and their nationalities: Chinese 154  Malaysia 38 India 5 Indonesia 7 Australia 6 France 4 USA 3  New Zealand 2 Ukraine 2 Canada 2 Russia 1 Italy 1 Netherlands 1 Austria 1

KUALA LUMPUR/BEIJING - China deployed 21 satellites to scour its territory for a missing Malaysian jetliner, while Australia said it had drastically narrowed its sector of the search area but was still looking in an expanse of ocean the size of Spain and Portugal.

Malaysia said on Tuesday it had conferred with the US and Chinese ministers on the search for Malaysian Airlines Flight MH370, an unprecedented operation involving 26 partners that now spans Asia from the Caspian Sea to the southern Indian Ocean.

Investigators are convinced that someone with deep knowledge of the Boeing 777-200ER and commercial navigation diverted the jet, 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 crash or hijack the plane, Western security sources and Chinese authorities said.

China has begun to search for MH370 in Chinese territory, which falls within the northern search corridor, said Xinhua News Agency, and Foreign Ministry spokesman Hong Lei told a daily news conference that 21 satellites were involved.

"In accordance with Malaysia's request, we are mobilizing satellites and radar to search over the Chinese section of the northern corridor which the Malaysians say the plane may have flown over," he said.

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."

STILL LOOKING FOR MOTIVE

China's ambassador to Malaysia said his country had carried out a detailed probe into Chinese passengers aboard the flight, which vanished on March 8, and could rule out their involvement.

"The probe into the incident's cause is not suitable to be conducted in a high-profile way," Ambassador Huang Huikang told Chinese reporters, state television said on one of its official microblogs.

US 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 US inquiries said the pilots were being studied because of the technical knowledge needed to disable the aircraft's communications systems.

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.

Flight MH370 vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur.

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, crossing the Malay Peninsula and following a commercial aviation route towards India.

Thailand said on Tuesday a re-examination of its military 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.

	Video: How can an airplane disappear? MH370 is not the first aircraft that has disappeared without a trace.
	Video: Officials remain puzzled Tension mounted as the search for the missing Malaysian airplane continued.