Intensive background checks of everyone aboard a missing Malaysian jetliner 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.
Malaysia said it had conferred with the US and Chinese ministers on the search for Malaysian Airlines Flight MH370, an unprecedented 26-nation operation 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 course.
Read: Planning could hold key to disappearance of jet
Malaysia plane's flight path changed by cockpit computer: report
China's ambassador to Malaysia said the country had carried out a detailed probe into its nationals 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 on Tuesday.
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.
HT Special: Mapping the world's best known hijacks
Search and rescue efforts on for the missing jet (AFP photo)
Focus on pilots
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 on a scheduled flight to Beijing.
Investigators piecing together patchy data from military radar and satellites believe that someone turned off 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.
Read: Malaysia police probe flight engineer
India suspends search for missing Malaysian jet MH370, awaits new instructions
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.
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 international 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 US 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."
Some US officials have expressed frustration at Malaysia's handling of the investigation. The Malaysian government still had not invited the FBI to send a team to Kuala Lumpur by Monday, two US security officials said.
China has also repeatedly voiced impatience with Malaysia's efforts.
Malaysia's Defence and Acting Transport Minister, Hishammuddin Hussein, told Reuters the country was co-operating with the FBI.
"I have been working with them," he said on Tuesday. "It's up for the FBI to tell us if they need more experts to help because it's not for us to know what they have."
Hishammuddin added that he had spoken to US Defense Secretary Chuck Hagel and "my counterpart in China" about the search for the plane, now in its 11th fruitless day.
Read: MH370's pilots investigated as fears of '9/11 style plot' grow
MH370 co-pilot had planned to marry girlfriend
China has begun to searching for the missing plane in Chinese territory, which falls within the northern search corridor, said state news agency Xinhua on Tuesday.
Australia, which is leading the southernmost leg of the search, said aircraft had made two sweeps of the southern corridor so far, and would make another sweep later on Tuesday.
The US Navy is sending a P-8A Poseidon, its most advanced maritime surveillance aircraft, to Perth, in Western Australia, to assist with the search.
Full coverage: Malaysian Airlines Flight MH370