Stalin had always been worried about the potential of the Leningrad Party to oppose him ever since the days of Zinoviev in the 1920s and Kirov in the 1930s.
The seize and importance of Leningrad and its distance from Moscow made it ripe in Stalin's mind to become a centre of opposition.
When Zhdanov died in 1948, Stalin encouraged Beria and Malenkov to purge the party of anyone deemed to be of dubious loyalty.
The two leading party officials in Leningrad - Voznesensky (a Politburo member) and Kuznetsov were arrested along with thousands of other officials.
They were forced to confess, tried in secret and executed.
Their death warrants had already been signed by the Politburo before their trial.