A Russian Metrojet flight crashed on the Sinai Peninsula in Egypt on Saturday, killing all 224 people aboard, and leaving investigators scrambling for answers.
By Tuesday, at least 10 passengers' remains had been identified after investigators combed through an eight-square-mile area of wreckage.
Gathered below are eight theories about what could have felled the airliner, some of which officials say they've ruled out.
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— Late Monday, a senior defense official told NBC News
that American infrared satellites did not detect a heat trail in the vicinity of the plane when it began to drop from the sky, an indicator that would suggest a missile had hit it. However, officials shot that down, saying, "The speculation that this plane was brought down by a missile is off the table."
2. On-board explosion
— A second U.S. defense official told NBC on Monday that surveillance satellites detected a "flash or explosion" in the vicinity of the falling plane on Saturday, suggesting an on-board explosion. "The plane disintegrated at a very high altitude," said the official. "There was an explosion of some kind." It is possible, however, that the heat flash had nothing to do with the plane, and was simply in the vicinity, the outlet reported.
3. Fuel-tank explosion
— On July 17, 1996, TWA Flight 800 exploded and crashed into the Atlantic Ocean after taking off from JFK airport in New York. A four-year investigation by the National Transportation Safety Board (NTSB) determined that the most likely cause was a fuel-tank explosion, however many conspiracy theories suggest it was something else. In 2006, a Transmile Airlines 727-200 in India was repositioning on the runway when the fuel tank in its left wing apparently exploded, prompting the U.S. NTSB to send officials to assist in the investigation. While such explosions are rare, the possibility remains that a fuel tank explosion could have downed the Russian Metrojet, however no evidence has yet emerged to verify or disprove such a possibility.
— An on-board explosion often points to an explosive device being placed or carried into the plane. There is a long list of airliner bombings and attempted bombings, a list that includes the 2004 Islamist terrorist bombings that crashed two Russian planes. No evidence has emerged to verify or disprove the possibility that a bomb felled the Russian Metrojet on Saturday. Members of ISIS claimed responsibility for downing the plane on Saturday, however U.S. officials said the group does not possess surface-to-air missiles. It is possible ISIS carried out the attack in some other way, however, like using a bomb.
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5. Mechanical sabotage
— Robert Galan, a French aviation expert and historian of airline disasters, told Al Jazeera
that sabotage would most likely require familiarity with the electrical or fuel systems of the Metrojet A320-200, and added that a bomb would likely be easier to use to crash the plane.
6. Mechanical failure
— The Associated Press reported
that the Metrojet airplane that crashed was 18 years old and suffered damage to its tail in 2001 when it grazed the ground during a landing in Cairo. But Alexander Smirnov, deputy general director of Metrojet, said Monday that, "We rule out a technical fault of the plane or a pilot error. The only possible explanation could be an external impact on the airplane." Russian officials, however, said that the investigation was ongoing, and that such conclusions had no evidence and were premature. "Much more work will have to be done on a detailed study of the plane’s constructive elements; flight recorders will have to be deciphered and analyzed," said, Alexander Neradko, the head of the federal Air Transportation Agency, according to The New York Times
7. Pilot error or sabotage
— Earlier this year, 27-year-old co-pilot Andreas Lubitz appeared to have crashed Germanwings Flight 9525 intentionally, and several news outlets reported that he rehearsed the descent on previous flights. Despite statements from Metrojet representatives, no evidence has yet emerged to verify or disprove that pilot error or sabotage felled Saturday's flight over Egypt.
— It has been reported that only the U.S., U.K., China, France, India, Israel, and Pakistan possess drones capable of delivering missiles. No evidence has emerged to verify or disprove that an unmanned aircraft delivered an explosive to the Metrojet flight. If a drone were present near the aircraft, it would likely be detected by satellite, radar, or other technology.
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