Yuri Verbitsky and his bodyguard were shot dead by unknown assailants in an apartment block at Peredovikov Prospect.
Police said that Mr Verbitsky was one of alleged Tambovsky Gang boss Victor Gavrilenkov's closest partners.
Mr Gavrilenkov was wounded and his two police bodyguards were killed when two gunmen opened fire with AKS-74 Kalashnikov assault rifles in the Nevskij Palace's Vienna Cafe on February 26.
Edinburgh, Scotland, attorney John Hyden, who was sitting alone in the cafe drinking coffee, was struck by three stray bullets and died immediately.
Mr Gavrilenkov was later released from the hospital because police lacked evidence to arrest him, said Anatoly Kostin, deputy head of St Petersburg's criminal police.
Police believe that the Vienna Cafe shooting occurred when Mr Gavrilenkov was waiting for an appointed "strelka," criminal slang for a meeting of opposing crime gangs to discuss some issue.
Last summer, Mr Gavrilenkov's brother was killed near his home on Moskovsky Prospect.
Mr Kostin alleged that the Gavrilenkov brothers organized their own arrangement within the Tambovsky gang about a year ago, called the "Stepanychi Group" (according to their patronymic), which controlled some Scandinavian and Russo-Finnish joint ventures. (SPP)