Six people were injured as a result of the blast on the evening of February 18 at the restaurant on Nevsky Prospekt, St. Petersburg's main street.
One of the injured, a 48-year old woman, remains in hospital.
The explosive device is reported to have partially destroyed the ceiling and shattered a number of windows in the restaurant.
St. Petersburg Governor Valentina Matviyenko described the blast as a "monstrous act of vandalism."
She said the explosion was likely an act of "hooliganism" rather than terrorism.
Police are checking other fast-food restaurants in the city for explosives.
(compiled from agency reports)