The co-pilot of the missing Malaysian flight spoke the last words heard from the cockpit, the airline's chief executive said. He reportedly said, "Alright, good night".
No trace of Malaysia Airlines Flight MH370 has been found since it vanished on March 8 with 239 people aboard.
A search of unprecedented scale involving 26 countries, is going on, covering an area stretching from Caspian Sea in the north to the southern Indian Ocean.
Airline chief executive Ahmad Jauhari Yahya 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, said a Reuters report.
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.
The last transmission from the ACARS system - a maintenance computer that relays data on the plane's status - was received at 1.07 a.m. 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 disappeared from radar less than an hour after taking off from Malaysian capital, 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 Datuk Seri Hishammuddin Hussein said: "We are looking at it." But it was only one of the possibilities under investigation, he said.