MOSCOW, Russia (CNN) -- Three people were killed when a car blew up near a subway station in Russia's second-largest city, St. Petersburg, on Tuesday, authorities said.

The victims included a 3-year-old child, an emergency worker told CNN. There was no immediate indication of what caused the explosion.

The blast took place in the city's Viborsky district, the worker said. Officials suspect a grenade may have caused the blast.