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.
"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.
Information from the FMS is among the data transmitted by the Aircraft Communications Addressing and Reporting System (ACARS) which sends information back to the airline's maintenance base.
That system later stopped working. It is not clear whether it shut off before or after Flight 370's last verbal contact with the ground, Malaysia Airlines CEO Ahmad Jauhari Yahya told reporters Monday.
Sources tell NBC News that whoever turned the plane around programmed the FMS and knew exactly what they were doing.
"This would be a very elaborate scheme," said Ross Aimer, a retired United Airlines pilot who few the Boeing 777. They would've needed "very, very extensive training to pull this off," he added.
Meanwhile, authorities continued to scour for the vanished aircraft.
The search area has grown to a massive 2.24 million square nautical miles, Malaysian officials said Tuesday. It has been divided into a 14-section grid, with Australia, China, Indonesia and Kazakhstan spearheading efforts in those areas to which they are closest.