Updated at 12:10 p.m. ET
An explosive device ignited in a metro car in St. Petersburg, Russia, Monday afternoon, according to Russian officials. Ten people were killed in the blast, Health Minister Veronika Skvortsova said.
St. Petersburg Gov. Georgy Poltavchenko said 43 injured people were taken to hospitals after the explosion in Russia's second-largest city.
Investigators say the blast was caused by a "homemade explosive device that they found in an abandoned bag in a metro car," reporter Charles Maynes told NPR. "Special services here also claim that they have surveillance footage of what they say is the suspect," he added.
It happened as Russian President Vladimir Putin was in St. Petersburg attending a pro-Kremlin media forum, Maynes reported for NPR.
"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, praising the train driver.
Police also found an explosive device at a different metro station that failed to detonate, Maynes added.
Officials said the device found in the station at Vosstaniya Square was equipped with shrapnel elements, 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 standing 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 balls — presumably ones that had been packed around an explosive.
TASS posted a YouTube video with cellphone footage showing people walking through a smoke-filled station.
Initial reports had suggested two explosions had struck at the core of St. Petersburg — one in the Sennaya station and another at the Institute of Technology station that's one stop farther south. But the RBC media outlet reports that the train was moving between those stations when the explosion hit. Citing Interfax, RBC says most of the damage was limited to one train car.
"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 workers rushing 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":
"One is, of course, ongoing problems [Russia has] with the Northern Caucasus. They've had several wars in Chechnya, the breakaway republic, which has essentially gone much quieter in recent years, but I think there's a suggestion that that's a possibility.
"More likely, though, is ISIS, frankly. Russia of course went into Syria, where they've been combating ISIS in theory though many say they're also supporting the Syrian leader Bashar al-Assad."
Mourners lay flowers and lit candles at the Sennaya metro station, and Gov. Poltavchenko's spokesman said on Twitter that he has declared three days of mourning.
The U.S. Consulate in St. Petersburg issued an emergency message to American citizens in the city, telling them to avoid the area.
"Review your personal security plans; remain aware of your surroundings, including local events; and monitor local news stations for updates," the consulate statement read. "Maintain a high level of vigilance and take appropriate steps to enhance your personal security."
This is a developing story. Some things that get reported by the media will later turn out to be wrong. We will focus on reports from police officials and other authorities, credible news outlets and reporters who are at the scene. We will update as the situation develops.