A woman writes a message for passengers aboard a missing Malaysia Airlines plane, at a shopping mall in Kuala Lumpur, Malaysia. (AP Photo)
Malaysian Prime Minister Najib Razak addresses reporters about the missing Malaysia Airlines flight MH370 at Kuala Lumpur International Airport. (Reuters photo)
Colonel Do Duc Minh (3rd L), Vietnam Air Force's 370 Division's Chief of Staff, points at a map as he speaks to reporters about search ...
In this file photo, clouds hang over the North Sentinel Island, in the Andaman and Nicobar Islands. (AP Photo/Gautam Singh, File)
A Malaysia Airlines employee writes a message expressing prayers and well-wishes for passengers onboard missing Malaysia Airlines flight MH370 at Kuala Lumpur International Airport in ...
Crew members look outside windows from a Malaysian Air Force CN235 aircraft during a search and rescue operation to find the missing plane over the ...
A P3C patrol plane of the Japan Maritime Self-Defense Force sits on the taxi way at Sepang air force base, Malaysia. (AP Photo)
A woman wearing a mask against the haze walks past a board saying "Pray for MH370" in front of the Kuala Lumpur City Center (KLCC). ...
This handout photo taken shows the Royal Malaysian Navy's missile corvette KD Laksamana Muhammad Amin (front) and Royal Malaysian Navy's offshore patrol vessel KD Selangor ...
A tourist walks past a signboard saying "Pray for MH370" in front of the Kuala Lumpur City Center (KLCC). (Reuters photo)
Malaysian investigators are trawling through the backgrounds of the pilots, crew and ground staff who worked on a missing jetliner for clues as to why someone on board flew it perhaps thousands of miles off course, the country's police chief said.
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 told a news conference on Sunday.
No trace of the Boeing 777-200ER has been found since it vanished on March 8 with 239 people on board, but investigators believe it was diverted by someone who knew how to switch off its communications and tracking systems.
Malaysia has appealed for international 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.
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Search and rescue efforts on for the missing jet (AFP photo)
Malaysian authorities believe that as the plane crossed the country's northeast coast and flew across the Gulf of Thailand, someone on board shut off its communications systems and turned sharply to the west.
Electronic signals it continued to exchange periodically with satellites suggest it could have continued flying for nearly seven hours after flying out of range of Malaysian military radar off the country's northwest 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.
Pilots' homes searched
On Saturday, police special branch officers 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, Zaharie has been described by current and former co-workers as a flying enthusiast who spent his off days operating a life-sized flight simulator he had set up at home.
Read: Malaysian police search homes of pilots of missing jet MH370
Police chief Khalid said investigators had taken the flight simulator for examination by experts.
Earlier, a senior police official said the flight simulator programmes were looked at closely, adding they appeared to be normal ones that allowed players to practice flying and landing in different conditions.
Police sources said they were looking at the personal, political and religious backgrounds of both pilots and the other crew members. Khalid said ground support staff who might have worked on the plane were also being investigated.
A second senior police official told Reuters investigators had found no links between Captain Zaharie, a father of three grown-up children and a grandfather, and any militant group.
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.
Asked if Zaharie's background as an opposition supporter was being examined, the first senior police officer would say only: "We need to cover all our bases."
Malaysia Airlines has said it did not believe Zaharie would have sabotaged the plane, and colleagues were incredulous.
"Please, let them find the aircraft first. Zaharie is not suicidal, not a political fanatic as some foreign media are saying," a Malaysia Airlines pilot who is close to Zaharie told Reuters. "Is it wrong for anyone to have an opinion about politics?"
Co-pilot Fariq was religious and serious about his career, family and friends said.
The two pilots had not made any request to fly together.
Experts doubt militant groups involved
With no clear motive established as to why someone diverted the plane, Khalid said all possibilities - hijack, sabotage, or personal or psychological problems of someone on board – were being investigated.
Transport minister Hishammuddin said authorities had not received any ransom or other demand. "That makes it very difficult for us to verify whether it is a hijacking or terrorist," he said.
Southeast Asia's homegrown Islamist militant groups, such as Jemaah Islamiah (JI), which carried out the Bali bombings in 2002, have been quiet in recent years after security forces either arrested or shot dead numerous members.
Read: Did Indian radars fail to detect MH370?
Experts said they doubted the remaining militants had the skills or capabilities to carry out a complex hijacking.
"JI has not been involved with violence in the region since 2007," said Sidney Jones, director of the Jakarta-based Institute for Policy Analysis of Conflict.
"The other groups that are active in Indonesia, in trying to make terrorist plots, are all not very competent. I would be extremely surprised if any group from Indonesia, the Philippines or Malaysia itself would be directly involved."
Analysis of satellite data showed the last signal from the missing plane was at 8.11am, almost seven hours after it turned back over the Gulf of Thailand and re-crossed the Malay peninsula.
It's location was not pinpointed — experts could only determine that the plane could have been anywhere in either of two arcs: one stretching from northern Thailand to the border of Kazakhstan and Turkmenistan, or a southern arc heading from Indonesia to the vast southern Indian Ocean.
Hishammuddin said Malaysia had requested further satellite data from several countries, including the United States, China and France, to help with the search.
A source familiar with official US assessments said it was thought most likely the plane had headed south into the Indian Ocean, where it would presumably have run out of fuel and crashed. Air space to the north is much busier, and the plane would likely have been detected.
Countries contacted by Malaysia to assist in the search range from the former Soviet central Asian republics in the north to Australia in the south, along with France, which administers a scattering of islands deep in the southern Indian Ocean uninhabited except for a handful of researchers.
The Indian Ocean is one of the most remote places in the world and also one of the deepest, posing enormous challenges for efforts to find any wreckage or the flight voice and data recorders that would be key to solving the puzzle.
Full coverage: Malaysian Airlines Flight MH370