Accountability for Unsolved High-Profile Murders Abroad

I read that in Spain, the case concerning the murder of pilot Kuzminov was closed because no suspects could be found, even despite the theory of a contract killing. If, for example, foreign special services are suspected of involvement, but the investigation yields no results and the case is closed – can relatives or the state of Ukraine in any way influence the resumption of the investigation or achieve justice? What are the general options for action in such a situation for people interested in solving the crime?