Newsnights Richard Watson has been piecing together the story behind the murder of Alexander Litvinenko, eight years ago in London. He speaks to intelligence sources and key witnesses - about the motives for the killing, the role of the Russian state and the polonium trail left in London. \r<br>**********\r<br>Newsnight is the BBCs flagship news and current affairs TV programme - with analysis, debate, exclusives, and robust interviews. \r<br>\r<br>Subscribe to our YouTube channel for highlights and bonus videos \r<br>\r<br>Follow us on Twitter @BBCNewsnight for the latest updates on #newsnight \r<br>\r<br>Add us on Snapchat - our username is BBCNewsnight\r<br>\r<br>And follow on Facebook for our best material, an early heads up on whats coming up, and to join in our debates
