Police had earlier said they were investigating whether any passengers or crew on the plane had personal or psychological problems that might explain its disappearance, along with the possibility of a hijack, sabotage or mechanical failure.
The plane left Kuala Lumpur for Beijing early on Saturday morning, vanishing from civilian radar screens about an hour after take-off over the sea separating eastern Malaysia from the southern tip of Vietnam.
There was no distress signal or radio contact indicating a problem and, in the absence of any wreckage or flight data, police have been left trawling through passenger and crew lists for potential leads.
"Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money, you know, we are looking at all possibilities," Malaysian police chief Khalid Abu Bakar told a news conference.