So was the assassin a  Ukrainian citizen  or not? 

The story isn't exactly clear on that somewhat important detail. If he was Ukrainian it shouldn't take much for the country's effective, well trained  and  corrupt free police to  ascertain any connections he might have had  to Putin or anyone else for that matter.

Having said that there have been at least five suspicious  deaths of various political officials across the spectrum over the last year or so which the police have been unable to shed any light on.