The way the evidence was gathered is that the authorities in Russia have a strange way of thinking that nobody from the outside is ever going to look at what they've done, so they document everything. When Sergei Magnitsky was beaten on the last night of his life, there was a protocol written and signed by the people who were involved in the beating, just to confirm to their boss that they did their job properly. In Sergei's case file, there is every piece of paper showing that he was requesting medical attention and showing the judges rejecting his requests for medical attention.
On April 29th, 2014. See this statement in context.