The UK government believes the Russian flight that crashed in Egypt on Saturday, killing 224 people, "may well have been brought down by an explosive device," according to an official statement. The UK also announced on Wednesday that it would suspend inbound flights from Sharm el-Sheikh, a tourism-heavy town in Egypt's Sinai Peninsula, where the flight went down. ISIS has a presence in the Sinai Peninsula, and has repeatedly claimed responsibility for the Russian plane's destruction, though analysts have seen some reason to doubt these claims. The UK announcement doesn't confirm that ISIS brought down the Russian plane, but it is perhaps the most worrying sign yet that it might have.

What the UK announcement means

Up until now, there was no real evidence that ISIS had shot down the Russian plane, Metrojet Flight 9268. Russian and Egyptian investigators found no evidence that the plane had been struck by a missile, and independent experts were skeptical that ISIS had the capability to hit it with one.

However, it may be possible that the plane was brought down by a bomb brought on board before the plane took off from Sharm el-Sheikh. Reuters, citing Russian officials, reports that the plane likely broke apart in midair. According to CNN, a US military satellite detected a "midair heat flash from the Russian airliner before the plane crashed."

The UK's suspicions, then, are credible. According to the British statement, "A team of UK aviation experts [is] currently traveling to Sharm to make an assessment of the security arrangements in place at the airport and to identify whether any further action is required." They plan to be done tonight.

If this does turn out to be an ISIS attack, as the group has claimed, this would be a worrying sign. ISIS's military efforts, in the past, principally focused on taking and holding territory in the Middle East rather than executing dramatic transnational terrorist attacks. An attack on a Russian commercial airliner would be a serious escalation on that front.

According an ISIS statement reported on Saturday, the attack was retaliation for Russia's recent intervention in Syria. The attack, the group said, was "in response to Russian airstrikes that killed hundreds of Muslims on Syrian land ... you who kill will be killed."