ST. PETERSBURG, Russia — A bomb blast tore through a subway train deep under Russia’s second-largest city Monday, killing 11 people and wounding more than 40 in a chaotic scene that left victims sprawled on a smoky platform. Hours later, anguish and fear rose again when police found and defused a shrapnel-packed explosive device at another St. Petersburg station.There was no immediate claim of responsibility for the attack, which came while President Vladimir Putin was visiting the city, his hometown. In the past two decades, Russian trains and planes have been frequent targets of terrorism, usually blamed on Islamic militants.News reports initially said police were searching for two suspects, and Russian state television showed a photo of one suspect wearing what appeared to be a skullcap characteristic of Russia’s Muslim regions. However, the Interfax news agency later cited unspecified sources as saying police now suspect the blast was the work of a suicide bomber linked to radical Islamists.The National Anti-Terrorism Committee said it was looking for the “perpetrators and organizers of the terror attack.”St. Petersburg, a major tourist destination famed for its imperial palaces and lavish art museums, had been spared previous attacks.“From now on, I will be scared to take the subway,” said Marina Ilyina, 30, who brought flowers to the station where the train stopped after the bombing. “We in St. Petersburg thought we wouldn’t be touched by that.”The explosion occurred in midafternoon as the train traveled between stations on one of the city’s north-south lines.The driver chose to continue on to the next stop, Technological Institute, a decision praised by the Investigative Committee as aiding evacuation efforts and reducing the danger to passengers who would have had to walk along the electrified tracks.