> Might have been FSB, but there were plenty of other parties that really didn't like Litvinenko and could have used Russian Polonium either to blame the FSB or to tell the FSB not to fuck with them, because they had the means to blame them for random hits (or a million other possible reasons)

According to the book 'A Very Expensive Poison' by Luke Harding, the Polonium used in the Litvinenko assassination is only produced in one reactor in the world, located in Ukraine. It would have been almost impossible to obtain without FSB knowledge or collusion, leading investigators to believe that Putin had direct knowledge and/or approval.