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A file photo of the co-pilot. [Photo / icpress]

KUALA LUMPUR - 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 a 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 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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	Video: Officials remain puzzled Tension mounted as the search for the missing Malaysian airplane continued.