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ISIS Bomb May Have Downed Russian Jet, U.S. Official Says

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Evidence indicates a bomb brought down Russia's Metrojet Flight 9268 over the Sinai Peninsula last weekend, and U.S. investigators are focusing on ISIS operatives or sympathizers as the bombers, a U.S. official told NBC News on Wednesday.

Questions have swirled over whether foul play or terrorism may have downed the Metrojet-operated Airbus A321 since it crashed in Egypt on Saturday, killing all 224 people aboard. A group claiming to be affiliated with ISIS took credit for the crash via social media media Wednesday.

The U.S. official said investigators are looking at the possibility that an explosive device was planted aboard the plane by ground crews, baggage handlers or other ground staff at the Sharm el-Sheikh airport before takeoff. U.S. officials had earlier told NBC News that an intelligence scrub of the passenger manifest and the flight crew aboard the jet showed no one with suspected ties to any terrorist group.

Rear Adm. John Kirby, the Pentagon's senior spokesman, told reporters Wednesday that while investigators were still examining data, but he said, "We need to advise U.S. workers not to go to the Sinai."

The news tracks with a statement earlier Wednesday by British Prime Minister David Cameron, who said the plane could have been brought down by a bomb. Cameron issued a delay on U.K.-bound flights from Sharm el-Sheikh, saying he wanted U.K. aviation experts to assess security there before travel resumed.

British investigators haven't been among the teams investigating the crash, but Egypt's Aviation Ministry said data from the crashed plane's data recorder had been "extracted and validated."