Updated at 12:10 p.m. ETRussian investigators say an "explosive device" ignited and ripped through a train car as it was traveling in between metro stations in St. Petersburg, Russia, on Monday afternoon. A second, unexploded device was found at a different metro stop.At least 10 people were killed in the blast, Health Minister Veronika Skvortsova said on television. The St. Petersburg governor's office released a statement saying that 43 people were taken to hospitals after the explosion in Russia's second-largest city."The blast happened between two stations, but the driver made a right decision not to stop the train and took it to the next station so that passengers could evacuate and the injured could be helped," said spokesperson Svetlana Petrenko of the Investigative Committee of the Russian Federation, which is now in charge of the investigation."It is possible that this helped avoiding even more victims," Petrenko said in a statement. The blast occurred as Russian President Vladimir Putin was in St. Petersburg attending a pro-Kremlin media forum, Charles Maynes reported for NPR from Moscow. He added that "special services ... also claim that they have surveillance footage of what they say is the suspect."Officials said the second device was found in the metro station at Vosstaniya Square and that it contained shrapnel, according to Interfax. A photo posted by the REN TV news network shows what looks to be a repurposed fire extinguisher, its dismantled red cylinder sitting next to a sheath of ball bearings.Those details would seem to align with an emergency care official's description, also given to REN TV, of seeing victims at the Sennaya station who had been wounded by metal balls — presumably ones that had been packed around an explosive.Initially, news reports had suggested there were two explosions — one at the Sennaya station and another at the Institute of Technology station one stop farther south. Citing Interfax, the RBC media outlet says most of the damage was limited to one train car.TASS posted a YouTube video with cellphone footage showing people walking through a smoke-filled station."The causes [of the blast] are unclear, that's why it is early to speak about this now," Russian President Vladimir Putin said Monday, according to TASS, which quoted him as saying that "investigators are considering various theories, including those linked to terrorism."In a post on Twitter, the Russian leader vowed to take "all necessary measures to provide assistance to those affected" by the blast.The country's prime minister, Dmitry Medvedev, described the incident as a "terrorist attack" in a post on Facebook. The investigating committee said it is investigating the incident as a "terrorist act," though it adds that the investigation will also look into other possible explanations.Video posted on social media showed rescue workersrushing to the scene. An image posted by another Russian state news agency, RIA, showed a damaged metro car and scattered debris.The station was evacuated and seven other stations were closed in the city, TASS added. Police were in the process of questioning witnesses and metro employees, according to RIA.The explosion happened in central St. Petersburg, which is popular with tourists, Maynes reported. He said that while authorities have not suggested who, if anyone, is responsible, there are several "likely suspects":