By Anshuman Daga and Yantoultra Ngui

KUALA LUMPUR (Reuters) - The co-pilot of a missing Malaysian jetliner spoke the last words heard from the cockpit, the airline's chief executive said on Monday, as investigators consider suicide by the captain or first officer as one possible explanation for the disappearance.

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.

A search unprecedented in its scale is now under way for the plane, covering an area stretching from the shores of the Caspian Sea in the north to deep in the southern Indian Ocean.

Airline chief executive Ahmad Jauhari Yahya also told a news conference that it was unclear exactly when one of the plane's automatic tracking systems had been disabled, appearing to contradict the weekend comments of government ministers.

Suspicions of hijacking or sabotage had hardened further when officials said on Sunday that the last radio message from the plane - an informal "all right, good night" - was spoken after the tracking system, known as "ACARS", was shut down.

"Initial investigations indicate it was the co-pilot who basically spoke the last time it was recorded on tape," Ahmad Jauhari said on Monday, when asked who it was believed had spoken those words.

That was a sign-off to air traffic controllers at 1.19 a.m., as the Beijing-bound plane left Malaysian airspace.

The last transmission from the ACARS system - a maintenance computer that relays data on the plane's status - had been received at 1.07 a.m., as the plane crossed Malaysia's northeast coast and headed out over the Gulf of Thailand.

"We don't know when the ACARS was switched off after that," Ahmad Jauhari said. "It was supposed to transmit 30 minutes from there, but that transmission did not come through."

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FOCUS ON CREW

The plane vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur. Malaysian authorities believe that someone on board shut off its communications systems as the plane flew across the Gulf of Thailand.

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.

Asked if pilot or co-pilot suicide was a line of inquiry,

Malaysian Acting Transport Minister Hishammuddin Hussein said: "We are looking at it." But it was only one of the possibilities under investigation, he added.

Intensive efforts by various governments to investigate the backgrounds of everyone on the airplane had not, as of Monday, turned up any information linking anyone to militant groups or anyone with a known political or criminal motive to crash or hijack the aircraft, U.S. and European security sources said.

One source familiar with U.S. inquiries into the disappearance said the pilots were being studied because of the technical knowledge needed to disable the ACARS system.

Many experts and officials say while the jet's transponder can be switched off by flicking a switch in the cockpit, turning off ACARS may have required someone to open a trap door outside the cockpit, climb down into the plane's belly and pull a fuse or circuit breaker.

Whoever did so, had to have sophisticated knowledge of the systems on a 777, according to pilots and two current and former U.S. officials close to the investigation.

Malaysian 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.

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.

Some U.S. officials have expressed frustration at Malaysia's handling of the investigation. As of Monday morning the Malaysian government still had not invited the FBI to send a team to Kuala Lumpur, two U.S. security officials said.

The FBI, which has extensive experience in investigating airplane crashes, and other U.S. law enforcement agencies have indicated they are eager to send teams to Kuala Lumpur, but will not do so unless formally invited.

VAST SEARCH CORRIDORS

Police and a multi-national investigation team may never know for sure what happened in the cockpit unless they find the plane, and that in itself is a daunting challenge.

Satellite data suggests it could be anywhere in either of two vast corridors that arc through much of Asia: one stretching north from Laos to the Caspian, the other south from west of the Indonesian island of Sumatra into the southern Indian Ocean west of Australia.

Aviation officials in Pakistan, India, and Central Asian countries Kazakhstan and Kyrgyzstan - as well as Taliban militants in Pakistan and Afghanistan - said they knew nothing about the whereabouts of the plane.

China, which has been vocal in its impatience with Malaysian efforts to find the plane, called on its smaller neighbor 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 P-3C Orion aircraft his country has already committed.

Malaysian Transport Minister Hishammuddin said diplomatic notes had been sent 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 were also searching the southern corridor, he said, and U.S. P-8A Poseidon surveillance aircraft were being sent to Perth, in Western Australia, to help scour the ocean.

NORTH OR SOUTH?

Electronic signals between the plane and satellites continued to be exchanged for nearly six hours after MH370 flew 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 about 30 minutes after that last satellite communication, Ahmad Jauhari said.

Twenty-six countries are involved in the search, stretching across much of Asia.

A source familiar with official U.S. assessments of satellite data being used to try to 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.

(Additional reporting by Niluksi Koswanage, Al-Zaquan Amer Hamzah, Stuart Grudgings and Anuradha Raghu in Kuala Lumpur, Mark Hosenball in Washington, Ben Blanchard and Michael Martina in Beijing and Sanjib Kumar Roy and Nita Bhalla in Port Blair, India, Sruthi Gottipati in Visakhapatnam, India, Frank Jack Daniel, Sanjeev Miglani and Douglas Busvine in New Delhi, Jibran Ahmed in Peshawar, Pakistan, and Raushan Nurshayeva in Astana and Olga Dzyubenko in Bishkek; Writing by Alex Richardson and Jim Loney; Editing by Nick Macfie and Ross Colvin)