Sometimes "innocent until proven guilty" needs to be put on the shelf. This is one of those times. The three people who Litvinenko met with (in 2 different meetings) shortly before developing the symptoms of poisoning should be in custody. Country club style custody, as none of them are yet proven guilty, and it's likely that 1 or 2 of them never will be. But it is really essential that the guilty party(ies) be identified, and then interrogated (using whatever means are necessary to extract the whole truth). The only way to do this is to grab the reasonable suspects NOW. Any of them who actually were in on the poisoning have a short life expectancy as it is, since there are certainly powerful people who want to make sure they can never tell what they know, and who have no qualms about killing people to achieve their objectives.