I'm beginning to wonder. Putin, and the old KGB apparatus in general, are a great many things. Stupid is not one of them. To pull off another poisoning while the last one is so centered in the spotlight would be dumb. The poisoning of Georgy Markiv, for example, was almost perfect -- it was detected only because of a technical glitch (the ricin pellet didn't dissolve like it was supposed to) and an extremely astute London medical examiner who spotted a tiny pin prick that would most often have been overlooked.
Unless Putin is planning to consolidate absolute power soon enough that he doesn't think he needs to worry about public opinion any more. That's the scary thought.
I have to remain open to the possibility that these poisonings are the work of others trying to undermine Putin -- not because I believe him when he says so, but because it makes sense. I know if I were a Russian with a rival I'd like to kill off, now would be the time, because the initial suspicion would fall far from me.
Not that there's anything wrong with that! ;-)