At least 10 people were killed and 50 others injured in a subway bombing on Monday in Russia's St. Petersburg, Russian press reports confirmed. The bombing hit two metro cars at two metro stations. Russian President Vladimir Putin who was in St. Petersburg for an event, visited the blast scene. He said it was early to determine whether the explosion stemmed from a terror attack. "We are investigating all possible causes of the bombing, including terrorism, "he was quoted as saying. Russian General Prosecutor's office is currently probing the incident. Three subway stations were closed immediately after the attack. Photos of the explosion that went viral on social media showed a smoke-filled platform, a damaged subway train and people lying on the ground, according to USA Today. All commuters were evacuated, state-run Sputnik news agency reported. St. Petersburg is Russia's second largest city.