It's not definitive that ISIS bombed a Russian charter jet out of the sky over the Sinai Peninsula, but if it did, the incident dramatically raises the stakes for the U.S.-led coalition in its battle against the Islamist terrorist group, security experts said.
Among the likely scenarios if it turns out that ISIS has developed the capability to target planes is that Russia would sharply increase its military presence in Syria, where it says it has declared war on the organization.
Until Wednesday, U.S. officials had said there was no hard evidence that ISIS or another terrorist group brought down the Metrojet-operated Airbus A321 over Egypt on Saturday, killing all 224 people on board.
Officials specifically dismissed claims by ISIS that it shot the plane down with a missile. But ISIS might not have had to use missiles — U.S. officials told NBC News on Wednesday there was significant evidence leading to "great confidence" among government analysts that a bomb loaded before takeoff was the actual mechanism.
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That, experts said, would mean ISIS will have proved that it has not just ambition but also real reach.
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U.S. officials continued to stress that it's still too early to conclude that for certain that ISIS bombed the jet, telling NBC News that mechanical failure remains a possibility.
But they said it was "likely" that a bomb was on the plane, probably planted by ground crews, baggage handlers or other ground staff at the Sharm el-Sheikh airport.
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At least three top officials at the airport, including its chief of security, were fired Wednesday after serious security lapses were discovered, Egyptian officials told NBC News.
And U.S. officials said investigators are beginning to focus on ISIS operatives or sympathizers as the likely bombers who exploited those weaknesses.
If ISIS was able to plant a bomb on the plane, "it's a real game-changer for the region," demonstrating that at least some of ISIS' grandiose boasts about its capabilities are true, said NBC News national security analyst Kevin Baron, executive editor of security analytics company Defense One.
The United States has already seen evidence that -- even as it has withstood more than 8,000 U.S. and coalition airstrikes -- ISIS has still managed to expand far beyond its base in Syria and Iraq, to Afghanistan, Egypt, Libya and even Nigeria and Yemen.
The group has a great deal of cash and a sophisticated online recruiting structure, making it capable of growing faster than al-Qaeda, which officials said has always been difficult to join. And combined with its "very aggressive" presence in the region, it now has a specific grudge against Russia, which began deploying special operations forces in Syria early last month.
NBC News national security analyst Michael Leiter, who was director of the U.S. National Counterterrorism Center during the administrations of Presidents George W. Bush and Barack Obama, agreed Wednesday not only that "the motive is there" but also that "the capability, I think, is largely there -- even if this is an upgrade."
U.S. officials told NBC News that the powerful state-run Russian media could quickly whip up nationalistic fervor in support of President Vladimir Putin's campaign in Syria.
That makes for a highly volatile situation given Putin's shoot-from-the-hip style, U.S. National Intelligence Director James Clapper said.
Putin sees himself as "a man on white horseback" riding in to save Syria, and he will likely respond by ramping up his forces there, Clapper said Monday at a Defense One summit in Washington, D.C. He said Putin would most likely increase the number of Russian advisers in Syria in the short term.
Regardless of the nature of the response, a senior U.S. military official in the region told NBC News on Wednesday, it's almost certain that Russia will retaliate "heavily and militarily."