There is still much that is mysterious about the fate of Malaysia Air Flight 370, but there is emerging consensus that the passenger jet bound for Beijing changed course, flying west over the Indian Ocean and flew for at least four hours. This tends to suggest that there was a human intervention, rather than a mechanical failure.
Typically such a human intervention would be a hijacking for political purposes, as was the case with the 9/11 flights or any number of other hijackings.
But no credible terrorist group has asserted responsibility for this operation and whoever diverted Malaysia Air Flight 370 issued no demands, which would be typical in the case of most hijackings.
There is always the possibility of pilot suicide, as was the case with EgyptAir 990, which plunged into the Atlantic shortly after leaving JFK Airport on October 31, 1999. The National Transportation Safety Board determined that the co-pilot intentionally caused the crash, although Egyptian authorities questioned that judgment.
But if Malaysia Air Flight 370 was a case of pilot suicide, why fly for so many hours and go through the trouble of switching off the transponder?
This leaves open the possibility that a person or persons with non-political or idiosyncratic motives commandeered the plane.
This is more common that one might presume. In the pre-9/11 era, before the introduction of reinforced cockpit doors, there were quite a number of such cases.
Air Safety Week, an aviation industry magazine, ran a story in January 2001 observing that, "the number of cases where the sanctity of the cockpit is threatened seems to be on the increase. In the last three years, there were at least 14 known attempts to enter the cockpit."
Air Safety Week pointed to a case from December 29, 2000, when a Kenyan man burst into the cockpit of a British Airways flight from London to Nairobi. The airplane plummeted more than 10,000 feet before the captain could regain control of the airplane.
Air Safety Week also pointed to two disturbing cases in the United States. On August 11, 2000, a 19-year-old Southwest Airlines passenger tried to get into the cockpit shortly before the plane was going to land in Salt Lake City. Six passengers restrained the man, who later died of a heart attack once he was taken off the plane.
On an Alaska Airlines flight on March 16, 2000, passenger Peter Bradley started taking his clothes off and ran up and down the aisle of the plane. The 39-year-old shouted at his fellow passengers "I am going to (expletive) kill you. I'm going to kill you all."
Bradley charged the cockpit door and then lunged for the flight controls. The co-pilot reached for an ax that was meant for emergency situations and began wrestling with Bradley. The episode ended when several passengers jumped on Bradley.
Since 9/11, plane commandeerings have continued despite the wide introduction of reinforced cockpit doors. Often such commandeerings are motivated by someone trying to claim asylum.
On January 24, 2007, there was a hijacking aboard Air West, a commercial airline flight in Sudan, which took off from the Sudanese capital, Khartoum. The hijacker Mohamed Abdu Altif entered the cockpit armed with a pistol around half an hour after takeoff and put the pistol to the captain's head. He demanded to go to London, but was told by the captain that there was not enough fuel, so he agreed to go to Chad. Altif apparently wanted asylum in the United Kingdom. The plane diverged to Chad where Altif surrendered to authorities.
Of course, if it's the pilot himself commandeering the plane in order to claim asylum, getting through the cockpit door is not an issue. A recent example of this occurred on February 17, when the co-pilot of an Ethiopian Airlines commercial flight between Addis Ababa, Ethiopia, and Rome took over the plane when the lead pilot went to the bathroom. The co-pilot flew the plane to Geneva, Switzerland, where he landed and asked for asylum.
Sometimes such a commandeering happens for reasons that are quite idiosyncratic. On October 4, 2006, Hakan Ekinci, slipped into the cockpit of a Turkish Airlines passenger airline flight between Tirana, Albania, and Istanbul, Turkey, after a stewardess had opened the cockpit door.
Ekinci claimed to have a bomb and that he would detonate it unless the plane was diverted to Italy so that he could speak to the Pope. Two Italian fighter jets scrambled and forced the plane to land in Brindisi, Italy.
Ekinci was a deserter from the Turkish military. He had written to Pope Benedict XVI during the summer of 2006 a letter asking for his help to avoid military service. He was arrested when the plane landed in Italy.
If Malaysia Air Flight 370 was indeed commandeered, the person or people responsible for it knew enough about civil aviation to know how to turn off the transponder. Of course, this is exactly what happened with three out of the four planes that were hijacked on 9/11 so it's a technique that is not unknown.
For the moment, the fate of Malaysia Air Flight 370 remains an enigma, but with the few facts that we now know about the flight, a commandeering of the passenger jet is the most plausible explanation.
Peter Bergen is CNN's national security analyst, a director at the New America Foundation and the author of Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad. Judd Smith attends Syracuse University School of Law.