Updated on Tuesday, March 18, at 7:59 a.m. ET.
Malaysia Airlines flight MH370 has been missing since Saturday, March 8, and every new piece of information seems to shroud the flight's disappearance in more mystery.
Malaysian investigators now say deliberate action was taken to turn off communications systems and steer the aircraft far off course. "Pings" sent from the plane to a commercial satellite hours after MH370 disappeared suggest either a northern or southern route of flight, creating a search area that stretches from Kazakhstan into western China or from Indonesia into the southern Indian Ocean.
The mystery has spawned dozens of theories from experts and armchair analysts alike, all with varying degrees of credibility. Going on the information made public so far, there are only a few theories that fit — though none satisfactorily. Here are the remaining likely possibilities for flight MH370. [5 Real Hazards of Air Travel]
1. Pilot suicide
MH370's transponder and its Aircraft Communications Addressing and Reporting System (ACARS) were both turned off shortly after the plane took off at 12:41 a.m. local time. There was a 14-minute gap between the last transmission of the ACARS and the last signal from the transponder, suggesting these systems were not destroyed by a sudden emergency. Furthermore a voice, thought to be the plane's co-pilot Fariq Abdul Hamid, spoke to air traffic control in Malaysia after the ACARS turned off and just before the transponder turned off. The final message from the lost plane was a calm, "All right, good night." (Update at 7:59 a.m. ET on March 18: This timing is now in question, as Malaysian authorities now say they are unsure exactly when the ACARS switched off, but it was between 1:07 a.m. and 1:37 a.m.)
The plane then turned from its Beijing-bound route. Military satellite detected it west of the Malaysian peninsula at 2:15 a.m. local time.
"Based on the details surfaced so far, it seems to have been a very well-planned and performed operation," said David Cenciotti, a former Italian Air Force pilot and journalist, who blogs at TheAviationist.com.
The expertise required to conduct these maneuvers has investigators looking into the pilot and co-pilot, Capt. Zaharie Ahmad Shah, 53, and 1st Officer Fariq Ab Hamid, 27.
Theoretically, one of these men could have decided to commit suicide by airplane. Pilot suicide would be highly unusual, but not unprecedented. For example, U.S. investigators concluded that a 1999 crash near Nantucket, which killed all 217 people onboard EgyptAir Flight 990, was the result of the co-pilot deliberately flying the plane into the sea (Egyptian investigators dispute that finding).
Similarly, SilkAir Flight 185, which crashed in Sumatra in 1997, may have been a pilot suicide. There were no mechanical failures to explain why the plane went into a vertical dive, the U.S. National Transportation Safety Board concluded. The plane's trajectory could be explained by the captain's deliberate action, however. [Understanding the 10 Most Destructive Human Behaviors]
Something similar could have happened to Flight 370. But the explanation seems strange: Other pilots who have committed suicide-by-plane have aimed the nose at the ground and ended it quickly. MH370 flew for hours after contact was lost.
"Why would you take 200 other people with you, a logical individual would want to know?" said Gregory "Sid" McGuirk, a professor of air traffic control at Embry-Riddle Aeronautical University.
2. Pilot conspiracy
Another theory holds that the pilots, or one of them, deliberately rerouted the plane for other reasons. This theory banks on the technical knowledge needed to change the plane's flight path, as well as suspicious circumstances around the flight's timing. [The 10 Craziest Conspiracy Theories]
Turning off the transponder and ACARS in the cockpit is as easy as flipping a switch and turning off a breaker, McGuirk told Live Science. The transponder, however, reportedly went off just as the plane was being handed off from Malaysian air traffic control to Vietnamese air traffic control. Radar only has a radius of 150 to 200 miles (240 to 320 kilometers), McGuirk said. Over the continental United States, radar overlaps so there are no gaps. Over the ocean, however, there can be no ground-based antennas. Some countries, including India, also have gaps in their radar coverage, McGuirk said.
"If that's the case, then somebody knew exactly where the radar coverage gap was and decided to act at that moment," McGuirk said. "That's kind of far-fetched."
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.
Editor's Note: This article was updated to reflect newly released information about the timing of the ACARS switch-off.
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.