Via The Times (UK):
Despite the comments of Yuri Chaika, the Prosecutor-General, intelligence services in Britain are convinced that the poisoning of Litvinenko was authorised by the Russian Federal Security Service.
Security sources have told The Times that the FSB orchestrated a “highly sophisticated plot” and was likely to have used some of its former agents to carry out the operation on the streets of London.
“We know how the FSB operates abroad and, based on the circumstances behind the death of Mr Litvinenko, the FSB has to be the prime suspect,” a source said yesterday.
Intelligence officials say that only officials such as FSB agents would have been able to obtain sufficient amounts of polonium-210, the radioactive substance used to fatally poison Mr Litvinenko only weeks after he was given British citizenship.