Still, the actions of the plane after the communications were turned off look deliberate, Cenciotti said. Programming the plane toward the navigational waypoints it appeared to be following wouldn't take too much expertise, he told Live Science. But the fact that the waypoints were so close to the edge of the Malaysian airspace boundary suggests they were deliberately chosen to obscure the plane's path.
"The farther to the radar, the harder to positively identify a so-called non-cooperative aircraft," Cenciotti said.
If one or both of the pilots did decide to reroute the plane, the motive they would have for doing so is unclear. Shah reportedly had strong political views and an at-home flight simulator; however, strong views do not necessarily suggest terrorism, and many pilots practice or play with flight simulators at home.
3. Terrorists Take Down Plane
The pilots also may have been forced by terrorists aboard the plane to disconnect the communications and change course, before crashing somewhere. Alternatively, whoever commandeered the plane could have been an expert in aircraft and flown it themselves.
Authorities have not ruled out terrorism as a cause; no groups have come forward to claim responsibility or make demands, however. Sometimes terrorist groups do stay mum. For example, when Pan Am Flight 103 was destroyed by a bomb over Lockerbie, Scotland, investigators spent three years before issuing arrest warrants for two Libyan men. In fact, it wasn't until 2003 that Libyan leader Muammar Gaddafi admitted the country's role in the bombing. [Dictator Deaths: How 13 Notorious Leaders Died]
4. Terrorists stash the plane
One possible explanation for why a terrorist group wouldn't claim responsibility for a hijacking: They plan to use the plane later.
The plane's northerly route may have taken it over remote areas where a Boeing 777 could potentially land — but landing a plane of that size without a functional runway would be difficult, particularly if the plane needs to fly again.
"It's very difficult to steal a 777 with Malaysian markings," McGuirk said. "It needs a 10,000-foot runway, so where are you going to put it down?"
Not to mention the eyebrows that would raise at a Malaysian airliner showing up where it shouldn't, McGuirk added.
If the plane flew north, which would give it a better chance of landing, it would have to fly over populated regions, making detection more likely. One far-out but not-impossible way to evade detection might be to "shadow" another plane, flying close so that both appeared to be the same object.
"It would be quite a difficult maneuver," Cenciotti said. "Let's not forget the entire maneuver, if performed, was performed at night, with no help from ground radars: Estimating reciprocal speeds, distances, altitudes based only on navigational lights is difficult. Maybe too much."
Beyond 9/11, there is precedent for the idea of stealing a plane for use in a later attack: In 1959, Brazilian Air Force officers hijacked a prop plane with 44 people aboard and landed it in southwest Brazil. They planned to use the plane in a bombing of Rio de Janeiro, but the plan fizzled and all hostages made it out of the ordeal alive. In 1994, Federal Express employee Auburn Calloway attempted to highjack a FedEx cargo jet for use in a suicide attack against the company's headquarters. The crew managed to overcome Calloway despite severe injuries. [9/11 Science: How Terrorist Attacks Rocked America]
Another plane, a Boeing 727-223, was driven off a runway in Angola in 2003. Aircraft mechanic Ben Charles Padilla and his employee John Mikel Mutantu were on the plane at the time, but it's not known whether they flew off in the aircraft or whether someone else killed them or took them hostage. The plane never reappeared, and the FBI closed the case in 2005.
5. Hijacking Gone Wrong
MH370's disappearance could also be linked to a hijacking gone wrong. In 1996, Ethiopian Airlines Flight 961 crashed in the Indian Ocean after hijackers demanded it be flown to Australia. The plane only had enough fuel to make it to its destination in Nairobi, but the hijackers refused to believe the pilots.
The pilots first tried to stay near the African coast, knowing they could never make it to Australia. When the hijackers insisted they steer east, pilot Leul Abate instead flew toward the Comoro Islands off the east coast of Africa. There, as the plane ran out of fuel, the pilots tried for an emergency landing at the airport on Grande Comore, but an attack by the hijackers forced them to ditch in shallow water. All but 50 passengers died.
Something similar could have happened on Flight MH370. Perhaps hijackers forced the crew to turn back toward Malaysia as part of a 9/11-like attack. If the crew fought back and all aboard ended up incapacitated, the plane could have continued flying on autopilot until it ran out of fuel.
All of these scenarios are speculative. "It all comes down to, 'What's the motivation?'" said McGuirk, adding that there are more questions than answers at this point.
"I've never seen a situation like this, where none of the theories seem to fit," he said.
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