UPDATED: An explosion at a subway station in St. Petersburg — Russia’s second largest city — left at least 10 people dead on Monday afternoon.

The blast, which occurred around 2:30 p.m. local time (4:30 a.m. PT), was reported to have happened in a subway carriage between the Sennaya Square and Technology Institute stations in the centre of the city. Early media reports had originally suggested two explosions had occurred, one at each station, but this was later amended by the Russia’s National Anti-Terrorism Committee. A spokesperson for the committee later said another device had been found and defused at Ploshclad Vosstaniya station.

According to the Associated Press, Andrei Kibitov, a spokesperson for the city’s governor, told Russian television that at least 10 people have been killed and another 50 injured in the explosions. All other stations in the city have been closed.

Moscow’s deputy mayor Maxim Liksutov told local news agency Interfax that authorities are tightening security on the Russian capital’s subway system. The National Anti-Terrorism Committee has announced security will be tightened at all critical transport facilities.

Russian President Vladimir Putin had been in St. Petersburg earlier in the day to attend the Truth and Justice media forum, which has been held in the city by the Russian Popular Front’s Truth and Justice Foundation since 2014. Although he says that the cause of the explosion remains unclear, the president has reportedly spoken to the heads of the Russian security services and told local news agencies that investigators would consider all potential causes, but would look at whether this was a terrorist attack “first of all.”

“Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened,” Putin said, ahead of a scheduled meeting with Alexander Lukashenko, the president of Belarus, who is in Russia on a working visit.

President Putin was born in St. Petersburg, when it was formerly called Leningrad, and served his first political appointment in the city’s mayoral office in the early 1990s.