The missing Malaysia Airlines jet's abrupt U-turn was programmed into the on-board computer well before the co-pilot calmly signed off with air traffic controllers, sources tell NBC News.
The change in direction was made at least 12 minutes before co-pilot Fariq Abdul Hamid said "All right, good night," to controllers on the ground, the sources said.
The revelation further indicates that the aircraft's mysterious turnaround was planned and executed in the cockpit before controllers lost contact with Flight 370. But it doesn't necessarily indicate an ulterior motive.
A woman reads messages expressing prayers and well-wishes for passengers on board missing Malaysia Airlines flight MH370.Manan Vatsyayana | AFP | Getty Images
"Some pilots program an alternate flight plan in the event of an emergency," cautioned Greg Feith, a former National Transportation Safety Board crash investigator and NBC News analyst.
"We don't know if this was an alternate plan to go back to Kuala Lumpur or if this was to take the plane from some place other than Beijing," the doomed flight's intended destination, Feith said.
Malaysian military radar last detected Flight 370 in the northern mouth of the Strait of Malacca, south of Phuket Island, Thailand, and west of the Malaysian peninsula — hundreds of miles off course.
Authorities said for the first time Saturday that Malaysia Airlines Flight 370 veered sharply off its flight plan because of "deliberate action by someone on the plane."
(Read more: Timeline of Flight MH370)
The course of the flight was changed by entering navigational instructions into the Flight Management System (FMS), the cockpit computer that directs the plane along a flight plan chosen by pilots.