smileless The warning was more than specific enough, for thie event to have been, if not totally avoidable, that there should have been a far far faster response.
I quote from the BBC report I linked in a post above
The 7 March warning from the US to its own citizens was unusually specific. It talked of reports that "extremists" had "imminent plans to target large gatherings in Moscow" and specifically mentioned concerts.
This warning was passed on to the Russian government as well. The dates were slightly out, but the Russians would probably have been given the data this warning was based on and added their own intelligence.
Any competent government would have followed it up, Had extra security at all big events at appropriate venues and had the relevant civil security group on standby.
They chose not to because it enabled Putin to lnk this event, quite deliberately with the Ukraine war. I suspect that he had hpped to blame Ukrainian ectremists, but there was so much information out there showing it was islamic terrorists, that he had to present it that they were fleeing to Ukraine that was expecting them, which, of course was not true.