At least 10 people were killed and 37 injured in a blast at St. Petersburg metro station Tekhnologichesky Institute on Monday.
Another explosive device was reportedly found at a different metro station and eventually dismantled.
Russia’s general prosecutor officially declared the incident a terror attack.
St. Petersburg authorities have called a three-day mourning in the city.
Russia’s President Vladimir Putin ordered authorities to provide assistance to the families of the killed in the explosion and to the wounded people.
An image of the suspect accused of planting one bomb in St Petersburg metro circulated on social media.
— Kom News (@KomNewsCom) April 3, 2017
No group has claimed the attack yet.