that russia s intelligence service, the fsb came behind it. the fsb operation to kill mr. litvinenko was probably approved by mr. petruchev, then head of the fsb, and also by president putin. reporter: in 2018, the same country, different poison. this time, former russian double agent sergei skripal and his daughter were poisoned by the nerve agent novichok. while the kremlin once again denied involved, putin not shy to show his contempt for skripal. translator: he is simply a spy, a traitor to his homeland. you get it? there s such a thing, a traitor to one s homeland. he s one of them. reporter: in 2020, the same poison, but this time inside russia. opposition figure alexier