I have yet to read the report, but I do remember, at the time, that there was a trail of polonium radio-activity which led to and from the restaurant to the hotel where the alledged perpetrators were staying and which also led to a plane returning them to and from Russia. At least one chain of causation has been established between the perps and the victim. Forensic sensors are so sensitive these days.
see url: http://www.bbc.co.uk/news/magazine-33678717
and url: https://en.wikipedia.org/wiki/Poisoning_of_Alexander_Litvinenko