After a Russian plane crashed in Egypt killing 224 people, a bomb stowed in the plane has become the most likely cause of Russia’s deadliest aviation disaster in the country’s history.

It is suggested that the bomb came from ISIS or an ISIS affiliated group, which may be revenge against Russia’s intervention in Syria. The plane broke up in midair and spread debris over 18 square kilometres, further supporting the theory that an explosive was inside the jetliner. Egypt’s civil aviation ministry retrieved the plane’s black box and has said that there is an apparent sound of an explosion in the recording. The U.S. military also reported picking up satellite footage of a heat flash before the plane crashed. A source at the Sharm el-Sheikh airport, where the plane took off, said that passengers must pass three or four security checks before boarding. This leads some to believe that the bomb was placed on the plane before takeoff and was not the result of getting past passenger security.

Initially the crash was believed to be a technical failure, as Russian airlines are four times more dangerous than the world average, mostly due to smaller airline companies. In 2011, nearly the entire Russian Lokomotiv Yaroslavl hockey team, including former NHL player Pavol Demitra, died in a plane crash northeast of Moscow.

According to Vladimir Putin’s spokesperson, the plane was only 18-years-old and was in good condition. The plane did receive damage on the tail in 2001 at Cairo airport that could have been incorrectly repaired, but Metrojet officials said the airliner has been regularly inspected since 2001.

Another theory suggests that a missile hitting the plane resulted in the accident. An Islamic militant group in Egypt claimed responsibility three times for the crash, but would not reveal the methods of how they brought it down. The plane was flying at 31,000 feet — a height at which analysts believe jihadist weaponry is incapable of functioning. Both Russian and Egyptian officials have dismissed the terrorism claims; possibly due to the damage it would cause Egypt’s tourism industry, and raising questions against Russian intervention in Syria.

Despite the unlikely possibility, Middle Eastern and European carriers have avoided flying over the region until the cause is confirmed. U.K. Prime Minister David Cameron has halted all flights between the U.K. and the Sharm el-Sheikh resort on the southern tip of the Sinai Peninsula in Egypt, leaving over 20,000 British citizens stranded.