Several senior U.S. officials have backed the theory that a bomb was behind the downing of Metrojet Flight 9268 in Egypt's Sinai Peninsula even as Russia began evacuating its citizens stranded in Egypt, according to reports.

Speaking to CNN, a U.S. security official said it was "99.9 percent certain” that the aircraft was the target of a terrorist attack while another said it was "likely." However Egyptian officials in charge of the main investigation have been wary of the bomb theory and have said that no conclusion has been reached yet. On Thursday, U.S. President Barack Obama too spoke of the possibility of a bomb being behind the crash.

"All the scenarios are out on the table," Ayman al-Muqaddam, the head of the investigation, told reporters Saturday.

British Foreign Secretary Philip Hammond also said Sunday that airport security may see an overhaul worldwide if the bomb theory is proven to be true, a sentiment that was echoed by Tim Clark, president of Emirates airline, according to Reuters

Clark also told reporters Sunday at the Dubai Airshow that Egypt is “an enormously important market” and that it plans to maintain its operations there.

Meanwhile Russian authorities have brought back 11,000 of the estimated 80,000 Russian tourists from Egypt in the last 24 hours after the Kremlin on Friday grounded all flights to the country due to security concerns.

Thousands of tourists, predominantly British and Russian nationals, are stuck at the Sharm el-Sheikh Airport, from where the ill-fated Metrojet flight took off for St. Petersburg, and crashed later in the Sinai region.

Metrojet Flight 9268 was travelling from the Egyptian resort town of Sharm el-Sheikh to St. Petersburg in Russia on Oct. 31, when it crashed, killing all 224 people onboard. Militants affiliated with the Islamic State group claimed responsibility for the attack and said it was in response to Russian airstrikes against ISIS fighters in Syria.