If a bomb on board planted by Isis or the Egyptian affiliate of the militant group did bring down the Russian airliner in the Sinai Peninsula last week, it would be very bad news for President Vladimir Putin, in both domestic and foreign policy terms.
Thus far US officials are stressing that it has not come to a firm conclusion that a bomb was responsible. But the fact that US intelligence – as well as its British counterpart – has let it be known it believes that is the most likely explanation is telling in itself.
The crash, in which 224 people died, came barely a month after Moscow began its direct military intervention in Syria. At the time Mr Putin said that Isis was a direct threat to Russia, but acknowledged that retaliation from Islamic extremists was possible. Now, it appears, that fear may have become reality.
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Until yesterday, the tightly controlled Russian media was non-committal about the cause of the disaster. Last night, however, a St Petersburg newspaper was said to be reporting that traces of explosive had been found on the bodies of some victim.
First shots of the crashed Russian plane in Sinai
Confirmation that a bomb – in all likelihood in retaliation for Moscow’s air strikes against Isis targets in Syria - did bring down the aircraft would for ordinary Russians transform the chaos in the Middle East from a distant headline into a painful immediate reality.
This in turn may damage Mr Putin domestically. For almost two years, the Russian leader has been hugely popular, riding a wave of nationalism fueled by his assertive actions in Ukraine, and now Syria. But proof that the plane was destroyed by a bomb might dent this aura.
Mr Putin could well therefore feel obliged to hit back even harder against Isis in Syria, and closer to home as well.
Some of the group’s recruits have come from the Islamic republics regions in southern Russia – most notably Chechnya where separatist and jihadist groups, some linked to Isis, still operate, despite a sustained crackdown by Moscow in the wake of the Chechen wars of the 1990s.