As I said before: few people are as good at committing assassination as the Russians. Western intelligence agencies refer to killing in the line of duty as, "wet work". That term actually dates back to Tsarist Russia, where Russian gangsters called murder, "wet affairs". The term was picked up by the KGB.
If the FSB wants someone dead, they are perfectly capable of killing him and making it look like an accident, suicide or natural causes. It's a bit like the scene from
Tomorrow Never Dies,
Bond, "If you shoot me from there, it won't look like suicide".
Dr Kaufman, "I'm a Professor of Forensic Pathology, Mr Bond. I could shoot you from
Sttutgart and make it look like suicide"