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There's significant evidence that a bomb brought down Russia's Metrojet Flight 9268 over the Sinai Peninsula last weekend, U.S. officials told NBC News on Wednesday, saying U.S. investigators are focusing on ISIS operatives or sympathizers as the likely bombers.

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. ISIS's media office in Sinai released an audio message Wednesday reiterating its claim of responsibility. Neither Wednesday's claim nor an earlier one immediately after the crash said how ISIS is supposed to have brought down the plane.

A U.S. official told NBC News he expects Russia to retaliate "heavily and militarily" if the theory is borne out.

U.S. officials stressed that while they believe it's "likely" that a bomb was on the plane, it's still too early to conclude that for certain. They told NBC News that mechanical failure remains a possibility.

Related: Investigators Examine Metrojet Black Boxes

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NBC News' terrorism intelligence partner, Flashpoint Global, said "it is unlikely that [ISIS] has the ability to have targeted and downed the airliner," noting that the group continues to release publicity about the incident without providing any video of its actually targeting the airliner.

But NBC News national security analyst Kevin Baron, executive editor of national security analytics company Defense One, said that if ISIS was able to plant a bomb on the plane, "it's a real game-changer for the region."

State Department spokesman John Kirby told reporters Wednesday: "We need to advise U.S. workers not to go to the Sinai." No U.S. commercial flights operate over Sinai, aviation officials told NBC News.

A 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. Passengers and the flight crew weren't significantly suspected after intelligence scrub of the passenger manifest and the crew showed no one with suspected ties to any terrorist group, officials said.

Three top officials at the airport, including the head of security, were fired Wednesday after investigators uncovered numerous lax security procedures, officials told NBC News.

The comments track with statements Wednesday by British officials.

Foreign Secretary Philip Hammond said after an emergency cabinet meeting Wednesday night that a review of "all the information we have available from a range of sources" led to the conclusion that "there is a significant possibility that the crash was caused by an explosive device onboard the aircraft."

Prime Minister David Cameron issued a delay on U.K.-bound flights from Sharm el-Sheikh, saying he wanted U.K. aviation experts to assess security there.

Related: Satellite Shows Heat Flash When Russian Metrojet Plane Crashed, But No Missile

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."