Did ISIS bomb Russian aircraft over Sinai?

British Prime Minister David Cameron weighed in on the investigation into the deadly Russian passenger jet crash Thursday. 

It is "more likely than not a terrorist bomb" that brought down a St. Petersburg-bound flight with 224 aboard in Egypt's Sinai Peninsula, British Prime Minister David Cameron said on Thursday. The Prime Minister's comment comes a day after British and US officials said that they have intel suggesting the plane was taken down by a bomb.

Russian and Egyptian officials have been adamant that the international community avoid jumping to conclusions while the investigation continues. In the meantime, Britain, Ireland, and the Netherlands have banned flights coming in and out of Sharm al-Sheikh, the resort area where Russia's MetroJet Airbus 321-200 originated. Germany has urged its citizens to avoid the Sinai Peninsula. 

US officials have told CNN that intelligence points to Islamic State (IS) or its affiliates placing a bomb on the Russian plane, which flight data shows broke apart in midair, leaving no survivors. Those officials emphasized that the US intelligence community has drawn no official conclusions, nor have US officials examined forensic evidence from the crash site.

Through intelligence, the unnamed US official told CNN that evidence suggests an individual at the Sharm el-Sheikh airport assisted in getting a bomb onto the plane.

"This airport has lax security. It is known for that," the official said. "But there is intelligence suggesting an assist from someone at the airport."

Egyptian Civil Aviation Minister Hossam Kamel pushed back on this theory altogether, saying there is no evidence that a bomb caused the catastrophe.

Prior to the crash, the US, Germany, and Britain had overflight warnings in place for Egypt's Sinai Peninsula, the Associated Press reports.

Germany filed its warning with the UN's International Civil Aviation Organization on Oct. 5, a declaration that is in effect until 2016, which is the same for both the British and American warnings. 

The AP reports that "the warnings advised airlines to avoid flying over the Sinai Peninsula below 26,000 feet and to avoid the Sharm el-Sheik airport due to extremist violence and, notably, the use of anti-aircraft weapons with what the US Federal Aviation Administration described as having the potential to reach high altitudes."

Egypt's civil aviation authority responded on Oct. 15 with a statement that claimed "all necessary measures for safeguarding the airspace are already taken from our side."

IS has been vocal about its responsibility for the crash. But those public statements are not what is leading US officials to believe Islamic State is responsible. The unnamed source speaking to CNN claims there are internal messages within the group that suggest IS involvement.

An Islamic State-affiliated group known as Province of Sinai released a statement quickly following news of the crash, saying it had carried out the attack "in response to Russian airstrikes that killed hundreds of Muslims on Syrian land," according to Reuters.

Russia began airstrikes in Syria in September in support of the current regime, but US officials have been skeptical that Putin is targeting IS directly. However, Col. Steve Warren, a spokesman for the US military's Operation Inherent Resolve targeting IS in Syria and Iraq, recanted, saying Wednesday that a small number of Russian airstrikes were hitting Islamic State targets in Syria.

"They've done hundreds of airstrikes at this point. I'm not putting out the count anymore, but they conduct airstrikes, but only a fraction of them have been against [ISIS] targets," he said. "And when I say fraction, I'm talking ... 10%."

In retaliation for US and Russian strikes, IS has called for war against both nations.