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The intelligence may not have come from inside Russia but from intercepted phone calls or messages from IS members based in the Middle East, or perhaps from a spy inside IS who learned of the general plan but not the detail.
Putin is so obsessed with his existential war with the West that he has chosen to ignore the threat posed by Islamist terrorists. His feeble claim that they fled to Ukraine after the attack will fool no one - especially not the hardliners he needs to keep on side.
I find the subtext is even more interesting: the US has access to intelligence concerning Russia’s state security that Russia itself doesn’t have; the intelligence is so granular that they could specify concert venues as a target; and the intelligence is reliable enough that they publicly disclosed it weeks ago.