U.S. investigators suspect that Malaysia Airlines 3786.KU -4.08% Flight 370 stayed in the air for about four hours past the time it reached its last confirmed location, according to two people familiar with the details, raising the possibility that the plane could have flown on for hundreds of additional miles under conditions that remain murky.

Aviation investigators and national security officials believe the plane flew for a total of five hours, based on data automatically downloaded and sent to the ground from the Boeing Co. BA -0.99% 777’s engines as part of a routine maintenance and monitoring program…

U.S. counterterrorism officials are pursuing the possibility that a pilot or someone else on board the plane may have diverted it toward an undisclosed location after intentionally turning off the jetliner’s transponders to avoid radar detection, according to one person tracking the probe…

At one briefing, according to this person, officials were told investigators are actively pursuing the notion that the plane was diverted “with the intention of using it later for another purpose.”