A previous version of this story suggested that sources based their suspicion that Malaysia Airlines Flight 370 flew hundreds of miles beyond the point at which communication was lost on signals from engine monitoring systems. The theory was in fact based on an analysis of satellite signals. The story has been corrected.

U.S. investigators suspect that Malaysia Airlines 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.

Military personnel scan the sea aboard a Vietnamese Air Force jet taking part in a search mission for the missing Malaysia Airlines aircraft. AFP/Getty Images

Aviation investigators and national security officials believe the plane flew for a total of five hours based on data sent by the Boeing Co. BA, -3.81% 777’s satellite-communication link designed to transmit to the ground the status of some onboard systems.

That raises a host of new questions and possibilities about what happened aboard the widebody jet carrying 239 people, which vanished from civilian air-traffic control radar over the weekend, about one hour into a flight to Beijing from Kuala Lumpur.

Six days after the mysterious disappearance prompted a massive international air and water search that so far hasn’t produced any results, the investigation appears to be broadening in scope.

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.

The investigation remains fluid, and it isn’t clear whether investigators have evidence indicating possible terrorism or espionage. So far, U.S. national-security officials have said that nothing specifically points toward terrorism, though they haven’t ruled it out.

When planes just disappear

But the huge uncertainty about where the plane was headed, and why it continued flying so long without working transponders, has raised theories among investigators that the aircraft may have been commandeered for a reason that appears unclear to U.S. authorities. Some of those theories have been laid out to national-security officials and senior personnel from various U.S. agencies, according to one person familiar with the matter.

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.”

As of Wednesday it remained unclear whether the plane reached an alternate destination or if it ultimately crashed, potentially hundreds of miles from where an international search effort has been focused.

In those scenarios, neither mechanical problems nor pilot mistakes nor another type of catastrophic incident caused the 250-ton plane to mysteriously vanish from radar.

An expanded version of this report appears on WSJ.com.

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