No trace of Malaysia Airlines Flight MH370 has been found since it vanished on March 8 with 239 people aboard. Investigators are increasingly convinced it was diverted perhaps thousands of miles off course by someone with deep knowledge of the Boeing 777-200ER and commercial navigation.
Suspicions of hijacking or sabotage hardened further after it was confirmed the last radio message from the cockpit - an informal "all right, good night" - was spoken after someone had begun disabling one of the plane's automatic tracking systems.
The last words were believed to have been spoken by the co-pilot, the airline's top executive said on Monday. "Initial investigations indicate it was the co-pilot who basically spoke," Malaysia Airlines CEO Ahmad Jauhari Yahya told a news briefing.
But police and a multi-national investigation team may never know for sure what happened aboard the jetliner unless they find the plane, and that in itself is a daunting challenge.
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Satellite data suggests the plane could be anywhere in either of two vast arcs: one stretching from northern Thailand to the borders of Kazakhstan and Turkmenistan, or a southern arc from Indonesia into the Indian Ocean west of Australia. Kazakhstan has joined the hunt for the missing jet.
Kazakhstan, however, said on Monday it had not detected any "unsanctioned use" of its air space by any planes on March 8. The Malaysia Airlines jet could hypothetically have reached the Central Asian nation's air space, but it would have been detected there, the Kazakh civil aviation committee said in a detailed statement.
"Even if all on-board equipment is switched off, it is impossible to fly through in a silent mode," said the statement signed by the committee's deputy head Serik Mukhtybayev. "There are also military bodies monitoring the country's air space."
Malaysia Airlines planes had made nine regular flights to and from Europe over Kazakhstan's territory on March 8, Mukhtybayev said.
"Even hypothetically thinking, before reaching Kazakhstan's territory this plane would have had to fly over other countries along its route, where the flight zone is also closely monitored, so we would have received information from these countries," he added.
Authorities are not ruling out the northern corridor and are eager for radar data that might confirm or rule out that path. The northern search corridor crosses through countries including China, India and Pakistan - all of which have indicated they have seen no sign of the plane so far.
China, which has been vocal in its impatience with Malaysian efforts to find the plane, called on its smaller neighbour to "immediately" expand and clarify the scope of the search. About two-thirds of the passengers aboard MH370 were Chinese.
Australian Prime Minister Tony Abbott said he had spoken to Malaysian counterpart Najib Razak by telephone, and had offered more surveillance resources in addition to the two Orion aircraft his country has already committed.
"He asked that Australia take responsibility for the search in the southern vector, which the Malaysian authorities now think was one possible flight path for this ill-fated aircraft," Abbott told parliament. "I agreed that we would do so."
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Malaysia's transport ministry said in a statement on Monday it had sent diplomatic notes to all countries along the northern and southern search corridors, requesting radar and satellite information as well as land, sea and air search operations.
The Malaysian navy and air force was also searching the southern corridor, it said.
Focus on crew
The plane's disappearance has baffled investigators and aviation experts. It disappeared 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 northeast coast and flew across the Gulf of Thailand, someone on board shut off its communications systems and turned sharply to the west.
That has focused attention on the crew. Malaysian police are trawling through the backgrounds of the pilots, flight and ground staff for any clues to a possible motive in what they say is now being treated as a criminal investigation.
The last words from the cockpit of the missing plane were spoken as it was leaving Malaysian-run airspace and being handed over to air traffic controllers in Vietnam.
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The sign-off came after one of the plane's data communication systems, which would have enabled it to be tracked beyond radar coverage, had been switched off, Malaysia's Acting Transport Minister Hishammuddin Hussein said on Sunday.
"The answer to your question is yes, it was disabled before," he told reporters when asked if the ACARS system - a maintenance computer that relays data on the plane's status -had been shut down before the "all right, good night" sign-off.
It is not known who on board spoke those words, which were first revealed last week.
The informal hand-off went against standard radio procedures, which would have called for him to read back instructions for contacting the next control centre and include the aircraft's call sign, said Hugh Dibley, a former British Airways pilot and a Fellow of the Royal Aeronautical Society.
Investigators are likely to examine the recording for any signs of psychological stress and to determine the speaker's identity to confirm whether the flight deck had been taken over by hijackers or the pilot himself was involved, he said.
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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 on Saturday. Among the items taken for examination was a flight simulator Zaharie had built in his home.
Among the items taken for examination was a flight simulator Zaharie had built in his home.
A senior police official familiar with the investigation said the flight simulator programmes were closely examined, adding they appeared to be normal ones that allow users to practice flying and landing in different conditions.
A second senior police official with knowledge of the investigation said they had found no evidence of a link between the pilot and any militant group.
"Based on what we have so far, we cannot see the terrorism link here," he said. "We looked at known terror or extremist groups in Southeast Asia, the links are not there."
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Background checks are also being made on the 227 passengers on the flight, including aviation engineer Mohd Khairul Amri Selamat, a 29-year-old Malaysian who worked for a private jet charter company.
"The focus is on anyone else who might have had aviation skills on that plane," the second police source told Reuters. As an engineer specializing in executive jets, Khairul would not necessarily have all the knowledge needed to divert and fly a large jetliner.
North or south?
Electronic signals the plane continued to exchange periodically with satellites suggest it could have continued flying for nearly seven hours after moving out of range of Malaysian military radar off the northwest coast, following a commercial aviation route across the Andaman Sea towards India.
The plane had enough fuel to fly for a total of about seven-and-a-half to eight hours, Malaysia Airlines' Chief Executive Ahmad Jauhari Yahya said on Sunday.
Twenty-six countries are involved in the search, stretching from the shores of the Caspian Sea to the far south of the Indian Ocean.
Three French civil aviation experts who were involved in the search for an Air France jet that crashed in the Atlantic in 2009 arrived in Kuala Lumpur on Monday to join the growing international search and investigation team.
A source familiar with official U.S. assessments of satellite data being used to try and find the plane said it was believed most likely it turned south sometime after the last sighting by Malaysian military radar, and may have run out of fuel over the Indian Ocean.
The Malaysian government-controlled New Straits Times on Monday quoted sources close to the investigation as saying data collected was pointing instead towards the northern corridor.
Investigators were also looking at disused airfields in the region with runways capable of handling a large passenger aircraft such as the Boeing 777, the paper said.
The New Straits Times also said that the plane dropped to an altitude of 5,000 ft (1,525 metres) or lower, using a low-flying technique known as "terrain masking" to defeat civilian radar coverage after turning back from its scheduled flight path.
The reports could not be immediately verified.
Meanwhile, an Indian Navy ship involved in search operations for the missing Malaysia Airlines plane on Monday arrived at the naval base in Port Blair. India on Sunday had put on hold its search for the missing aircraft, at the request of the Malaysian government, which wished to reassess the week-old hunt for the Boeing 777.
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