Kirov was assassinated by Nikolayev on 1 December 1934.
Nikolayev appeared to be acting alone and he certainly had both the motive and opportunity to carry out the murder.
Nikolayev had been expelled from the party in Leningrad, in which Kirov was the head and his ex-wife worked as a secretary in Kirov's office and there were rumours that she was having an affair with him.
Among Nikolayev's possessions was a map of Kirov's route to work and a diary with a plan of the murder.