Could you present some evidence of what you claim, first? For a denial to exist, there must be a credible case providing evidence. But, I am only aware of allegations that include no evidence or conclusions.
The few cases I recall involved:
- one person being murdered on a bridge by the Kremlin, whose murderers were caught - a pair of Muslims from Chechnya who vowed revenge for some policies they supported
- Anna Politkovskaya, who wrote in an obscure paper and had received many death threats over the years (I don't know what the outcome of the investigation was)
- Alexander Litvinenko, who drank polonium tea. And, that's all that's know.
If you have some evidence that suggests a conclusion for any of these things, maybe you ought to present it to authorities, so that allegations can be made, and so that social media commentators can start working on their responses to those allegations. You claiming you 'just know it,' like the "Russia hacking" claims, is not evidence.