Ten people were killed and dozens injured in a blast on the Saint Petersburg metro system on Monday that authorities are investigating as an "act of terror."
Here is what we know about the blast in Russia's second city:
The blast took place at 2:40pm local time (1140 GMT) on an underground train travelling between the Technological Institute and Vosstaniya Square stations, located in the heart of the city centre.
Investigators said the train driver did not stop between stations, a decision that enabled a quick evacuation.
Authorities gave an initial death toll of around 10 dead but this could still rise further as dozens were taken to hospital with injuries.
Russia's national anti-terrorism committee (NAK) later confirmed that the security services had found another explosive device at the Vosstaniya Square metro station. This device did not explode and it was immediately "neutralised", the committee said.
Russia's Investigative Committee said it had launched a probe into an "act of terror" but insisted it would nonetheless investigate "all other possible versions of this incident."
President Vladimir Putin had said earlier Monday that authorities were looking into all possible causes for the blast but said that a terror attack was being probed "first of all".
Russian authorities have claimed to have foiled multiple terror attacks on transportation systems in Moscow, some by suspected jihadist cells.
And Russia's transport has been targeted before.
In 2013 the country was hit by twin suicide strikes on a station and a trolleybus that claimed 34 lives and raised alarm over security at the Sochi Winter Olympic Games.
If this latest blast is confirmed to be an attack, it will raise jitters around a year before Russia is due to host the 2018 football World Cup in 11 cities, including Saint Petersburg.
Emergency vehicles rushed to the scene and authorities immediately blocked off some of the city's main arteries surrounding the transport hub.
The metro network said it was shutting down completely and Moscow's metro announced it was stepping up security in light of the Saint Petersburg blast.
The NAK said that security was being boosted at transportation hubs and crowded places across the country.
Television pictures showed the door of a train carriage blasted out and bloodied bodies strewn across the platform.
Local authorities announced three days of mourning in the city while Putin, who was holding a meeting outside Saint Petersburg, offered "condolences" to those hurt in the blast and to the loved ones of those killed.
With heightened security measures, Russians are certain to face more checks at airports, train stations and in the country's metro systems.