The co-pilot of a missing Malaysian jetliner spoke the last words heard from the cockpit, the airline's chief executive said, 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, Reuters news agency reported.
A search of unprecedented scale involving 26 countries is under way, 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 on Monday that it was unclear exactly when one of the plane's automatic tracking systems had been disabled, appearing to contradict comments by government ministers at the weekend.
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.
That was a sign-off to air traffic controllers at 1:19 am local time, 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 - was received at 1:07 am local time, as the plane crossed Malaysia's northeast coast.
"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."
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.
Malaysian police are trawling through the backgrounds of the pilots, flight crew and ground staff for any clues to a possible motive in what is now being treated as a criminal investigation.
Asked if suicide by the pilot or co-pilot 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 said.
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, US and European security sources said.
One source familiar with US 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 that, 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 US officials close to the investigation.
Malaysian police searched the homes of the captain, Zaharie Ahmad Shah, 53, and first officer, Fariq Abdul Hamid, 27, in middle-class suburbs of Kuala Lumpur close to the international airport on Saturday.