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Just to clarify, I never suggested that the culprits in this case extracted polonium from brushes (or any other end-use product containing polonium); but that, if the brush manufacturers can obtain enough polonium to make a go of it, then anybody else could obtain material in quantity from the same sources.
Back in the day, Stan Owsley was able to purchase large quantities of mescaline from legitimate suppliers by setting up a fake lab. He even hosted federal inspectors in his lab, and got away with it. This sort of thing would, I think, be even easier to pull off in Russia, a nation many of whose bureaucrats rely on graft for their livelihoods.
The only people who had the technological skill to pull off this poisoning are to be found among the nuclear powers, and probably only one or two of them at that.
This is why this matters, isn't it? I dispute this. Litvinenko's killers could have purchased their material from a lab or a company, or from a large number of small sources. They could have bought it from a corruptible employee at a nuclear power plant. They could have devised a simple and effective delivery system, and done their murder.
The participation of a state actor is not required to explain this case. It is not yet possible to rule out a plot by rogue actors. It may still turn out that Putin did it. On the other hand, it may not.