On October 31, Metrojet Flight 9268 seemingly disintegrated over Egypt's Sinai Peninsula while en route from Sharm el-Sheikh, in Egypt's Sinai peninsula, to Saint Petersburg, Russia.The disaster resulted in the death of all 224 people aboard.ISIS' Egyptian affiliate, which is based in the Sinai, quicklyclaimed responsibility for the crash. And Western intelligence officials, led by the UK and the US, have stated that based upon terrorist chatter and other evidence, a bomb was likely the cause behind the plane's destruction. On November 9th, an unnamed US official told CNN that the US was "99.9% certain" that a bomb had brought down the plane.However, intelligence agencies haven't made public any definitive proof that supports thebomb theory. Although it's to be assumed that the US and UK assessments are based off persuasive evidence that's unavailable to the general public,other possibilities for the cause of the crash haven't been totally ruled out yet, and the investigation is ongoing.Here's where things stand with some of the leading theories on what brought down the plane.A bomb planted on the planeAlthough media reports have largely beenbased on anonymous sourcing, French, UK, and British intelligence officials all now state that they believe that a bomb was the most likely cause of the incident. President Barack Obama and British Prime Minister David Cameron have each lent credence to claims that a bomb brought down the plane without explicitly labeling a bombas the cause.Egypt has now also launched its own inquiry into the accident to see if a bomb caused the accident.In most news reports, anonymous officials from western intelligence agencies have based their belief that a bomb downed the plane upon "terrorist chatter" indicating that a plan to bring down a jetlier was in the works. However, an assessment from the The Soufan Group notesthatterrorists generally fill their communications with threats and possible plans in order todistract intelligence agencies, getting them to chase down fake operations that the groups have no actual plan on carrying out.The Wall Street Journal notes that ISIS' leaders in Syria and Iraq were seemingly surprised by the downing of the plane. If ISISactually did bomb the plane, it seems that the terror group's Egyptian affiliate did it without coordinating with the "Caliphate's" leadership.In addition to the potential terrorist chatter,the last second captured by the plane's cockpit voice recorder (CVR) reportedly includes a suspect burst of sound.The noise will be analyzed with spectral analysisin order to determine if it's relevant to the cause of the crash."[The noise of a bomb] is very sudden, very sharp. It has a very distinctive profile to it," Tom Haueters, an accident investigator and veteran former National Transportation Safety Boardofficial, told ABC News . "So you can tell bombs. Actually, they stand out."Finally, it's always possible that the information that's made one analyst "99.9% certain" of the bomb hypothesis hasn't been made public yetor that it's so classified and so sensitive to ongoing intelligence operations that it will never be made public.An inside jobSupportingthe theory that a bomb brought down the plane is the possibilitythat ISIS or some other group coordinated with someone working at the Sharm el-Sheikh airportto sneak a bomb onto the plane.ABC reports that electronic interceptsindicate that ISIS was in communication with an airport employee before the downing of the plane. And The Wall Street Journalreports that the Egyptian government is now questioning airport employees and reviewingsecurity footage in order todetermine if an employee aided in the attack.Egyptian security forces, the Journalnotes, have now also been deployed on the tarmac of the airport to ensure that no one can access aircraftwithout supervised approval. The Soufan Group notes that if a bomb was used to down the plane, the most likely scenario would be that an airport employee either placed the bomb into the plane's cargo hold, or gave access to an authorized person to place the bomb inside the plane.A technical defectThe BBC reports, citing the pilot's widow, that the pilot of the plane called home before the flight and said that the plane's condition"left much to be desired."Metrojet has said on November 5 that it will cease operating all Airbus A321 aircraft while it is carries out inspections of its fleet. Buton November 2, a top Metrojet official said that the only likely cause of the disaster was an "external impact" on the airplane.Metrojet has tried to distance itself from fault in the crash. But there's still compelling circumstantial evidence that the plane's condition resulted in the crash.TheBBC notes that the aircraftsustained a "tail strike," or an incident where a plane'stail collides withthe tarmac on landing, during a landing in Cairo in2001 which necessitated three months of repair. Theaircraft "suffered severe rear fuselage damage upon landing" after the incident, according to Aviation Daily.The aircraftreentered service in 2002.The plane's eventual destruction suggests a parallel to the crash of Japan Airlines Flight 123 in 1985. That plane, a Boeing 747, had also suffered a tail strike several years earlier. It crashed when the plane's rear-pressurebulkhead blew during the plane's pressurization.That bulkhead, which had been faultily repaired after the aircraft's earlier tail strike, creates an airtight seal between the interior of the aircraft and the environment outside, allowing the plane to remain pressurized even as theexternal air pressure changes during the plane'sascent. The loss of the bulkhead resulted in explosive decompression, and the plane crashed about 30 minutes after the first report of an emergency onboard, killing 520 people.In 2002, a China Airlines 747 crashed in the Strait of Taiwan 20 minutes after takeoff. That plane had suffered a tailstrike 22 years earlier, and the crash stemmed from a faulty series of repairs in the months after theincident.The parallels between the crashes aren't perfect. Even if all three aircraftcame apart at roughly similar points in their flight, the severity of the Metrojet airliner's tail strike still hasn't been reported. It would detract from the mechanical failure hypothesis if it turns out the plane's tailhad just lightly scratched the tarmac back in 2001.It's not unprecedented for a plane with a tail strike history to crash as the aircraft is pressurizing, and mechanical failure hasn't been totally ruled out.A missileThemost unlikely yet by far most alarmingscenario behind the destruction of Flight 9268 is that ISIS or another group launched a missile which hit the plane.The plane was flying at over 30,000 feet at the time that it began to disintegrate. Usually onlystatemilitaries have anti-aircraft weaponsthat could reach a plane at such a high altitude. Most Man-Portable Air Defense Systems (MANPADS), which terror groups could use with minimal training, have a maximum targeting range of, at most, 23,000 feet.This would placeFlight 9268 out of rangeunless ISIS had managed to acquire an anti-aircraft weapon similar to theSA-11 Buk missile system, which Russian-supported Ukrainian separatist militantsused to down Malaysia Flight MH17 over Ukraine.But the likelihood that terrorists without a dedicatestate sponsor would have such a long-range and capable anti-aircraft system is highly unlikely.There is some recent precedent for a missile nearly causing a disaster over the Sinai. In August, a Thomson Airways-operated flight to Egypt passed within 1,000 feet of a missile while over the Sinai. The plane managed to avoid the missile after taking evasive action.Britain's Department of Transport said that the missile was fired duringEgyptian military exercises. Flights over the Sinai, The Guardian reports, must fly over 26,000 feet to ensure that they are not vulnerable to potential missile attacks from militants operating in the area.Armin Rosen and Benjamin Zhang contributed to this report.SEE ALSO:The 'nightmare scenario' for the Russian plane crash in Egypt is still very much in playJoin the conversation about this storyNOW WATCH: Investigators say flight MH17 was struck by a Russian-made surface-to-air missile
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