I am so sick of this theme, and the chances of it being solved are slim, the chances that the solution will seem believable are even slimmer. But here is an interesting article from The Sunday Times (extract from a book by Martin Sixsmith):
The more I probed, the more I was becoming convinced that Litvinenko had been poisoned by a group of people independent of the Kremlin but with close connections to the Russian security forces.This essentially brings us back to why Litvinenko was in London in the first place. I am not providing further comments, I have provided them in an opinion in November of 2006. And it turns out I was not far off from the truth. It is very unfortunate that Mr. Litvinenko's death was used by Mr. Berezovskiy as a PR stunt. It is more regrettable that European and US officials fell into this obvious trap.
The sophisticated planning behind the plot and the evidence of the poison factory strongly suggested an FSB-style operation. This did not necessarily mean the FSB had been acting in its own interests or on its own initiative; the security forces were so fragmented and out of control in the years following the collapse of the Soviet Union in 1991 that individuals and groups within them were constantly taking on moonlighting jobs on behalf of paying customers.
The letter harked back to an unprecedented act of public defiance that Litvinenko organised nine years ago after he and five other officers in an FSB hit squad, known as URPO, had been ordered to assassinate Boris Berezovsky, the immensely powerful Russian billionaire.
Far from killing him, Litvinenko had alerted Berezovsky and, at his behest, had dragooned the URPO men into denouncing the assassination order at a press conference. The act of rebellion was quickly crushed and led ultimately to Litvinenko’s flight to London — and Berezovsky’s.
Litvinenko left a legacy of seething anger not only among the superiors he had denounced but among the URPO officers who believed he had destroyed their careers — and almost their lives — by dragging them into his revolt.