A bomb planted by IS or an affiliate of the group was the most likely cause of the Russian jet crash in Egypt, US intelligence suggests.
One US official said the bomb theory was not the official conclusion of the intelligence community, but it was a strong possibility.
"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," the official told CNN.
The Airbus A321, operated by Russian airline Metrojet as flight KGL9268, was carrying holidaymakers from the Red Sea resort of Sharm el-Sheikh to St Petersburg.
It crashed with 224 people on board in Egypt's Sinai Peninsula on Saturday after breaking up in mid-air.
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