The Saturday morning crash of a Russian passenger plane in the Sinai Peninsula, killing 224 people, was immediately claimed by the local ISIS affiliate, though both Egypt and Russia immediately denied this, and analysts are similarly not putting much stock in the idea.
Though everyone seems to agree that Russia would be a “high-value target” for ISIS, the question of how such an attack would be carried out remains unanswered. ISIS almost certainly wouldn’t have the sort of anti-aircraft missiles to fit a plane flying at such an altitude.
ISIS insisted that they used “brains more than explosives” for the downing, however, which could suggest some sort of sabotage or even the planting of a bomb on board the plane before takeoff. So far there isn’t a formal explanation for how the plane broke apart in mid-air.
Until there is a good explanation, that means the ISIS claim of credit is at least possible, though previous incidents of ISIS claiming credit for things they clearly had nothing to do with, like the attack on a museum in Tunisia, has led most to assume they’re just making the claim in the hope somebody believes them.