Putin's Syria plan may have come back to bite him
Yesterday's blast on the St Petersburg metro system was the worst terrorist attack outside the North Caucasus since two suicide bombers killed 32 people in Volgograd in 2013.
No group has yet claimed responsibility for the attack, and Russian investigators have not announced a motive.
But the immediate suspicion will fall on the same groups that hit Volgograd, Moscow, and other cities over the past two decades - Islamists radicalised by the insurgency that emerged from the brutal Chechen wars of the 1990s and 2000s.
If so, it will be viewed as a worrying sign that a scourge that had all but been eliminated is back.
For the past few years, that insurgency has been on the back foot - largely owing to a massive crackdown Russia's security services launched in the run-up to the 2014 Sochi Winter Olympics.
That campaign, it later emerged, included a covert plan to help problematic insurgents travel to the Middle East in exchange for a promise never to return.
It was remarkably effective. The Volgograd attacks did not herald a new wave of attacks, but seemed to be a last throw of the dice.
But it also contributed to the emergence of a powerful 'Russian' - actually former Soviet - jihadi movement in Syria.
Russian and other former Soviet citizens make up a large (by some estimates the biggest) proportion of the foreign fighters with Isil.
With a reputation for ferocity and discipline, the 'Russian' brigades established themselves in Mosul and reached senior positions within Isil.
Vladimir Putin justified his war in Syria partly as a way of killing those groups before they came home to wage jihad in Russia.
"We have to fight them over there so we don't have to fight them over here," was the basic message to the Russian public.
As the West has learnt from bitter experience, bombing countries thousands of miles away doesn't stop domestic terror.
But that doesn't mean yesterday's attack will be viewed as a defeat for Mr Putin's Syrian strategy.
Like many Western governments, the Kremlin will likely point to it as another reason why its own war on terror is justified. (© Daily Telegraph, London)