"By signing off from Malaysian airspace at 1.19 a.m. on March 8 with a casual "all right, good night," rather than the crisp radio drill advocated in pilot training, a person now believed to be the co-pilot gave no hint of anything unusual.

Two minutes later, at 1.21 a.m. local time, the transponder - a device identifying jets to ground controllers - was turned off in a move that experts say could reveal a careful sequence."

-Reuters

Turning off the transponder two minutes after communication and turning IMO rules out fire. It would have had to take place in a 2 minute window.