The blast, around 2:40 p.m. local time, occurred as Russian President Vladimir Putin was in St. Petersburg attending a pro-Kremlin media forum, Charles Maynes reported for NPR from Moscow.
"Video from the scene showed chaos — the doors of the subway car twisted outward and blood-soaked passengers scattered and dazed," Maynes reported. "After the blast, survivors pounded out the metro car windows and passersby pulled the injured onto the station platform as rescuers arrived on the scene."
As of midday Monday, Maynes reported, Russian special services said "that they have surveillance footage of what they say is the suspect."
Investigators now say they have issued warrants for two suspects, Maynes reported. One suspect allegedly placed the explosive device in the metro car and another is suspected of leaving the unexploded bomb.
Officials said that 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 victims who had been wounded by pieces of metal, balls and screws — 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 Technological Institute station one stop farther south. Citing Interfax, the RBC media outlet says most of the damage was limited to one car in the southbound train.
TASS posted a YouTube video with cellphone footage showing people walking through a smoke-filled station.
In a post on Twitter, Putin vowed to take "all necessary measures to provide assistance to those affected" by the blast. He was quoted by TASS as saying that in regard to motive, "investigators are considering various theories, including those linked to terrorism."
The country's prime minister, Dmitry Medvedev, described the incident as a "terrorist attack" in a post on Facebook. And the Investigative Committee of the Russian Federation said it is investigating the incident as a "terrorist act," though it adds that the probe will also look into other possible explanations.
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."
Video posted on social media showed rescue workers rushing to the scene of Monday's blast. An image posted by another Russian state news agency, RIA, showed a damaged metro car and scattered debris.
The Sennaya station was evacuated immediately after the explosion 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.
Mourners lay flowers and lit candles at the Sennaya station, and Gov. Georgy 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."