Russia claims that it has documentary evidence. But the representatives of the United States, France and Great Britain in the UN Security Council refused to consider them and blocked the investigation. This is a very handy strategy for dealing with inconvenient facts - just declare them to be Putin's propaganda and pretend that the facts have ceased to exist after that.
What investigation was blocked and how do you know Russian evidence was ignored? The international atomic energy agency just completed their instigation and said Russia is full of shit.