A 22-year-old born in the former Soviet republic of Kyrgyzstan has been accused of carrying out Monday's attack on a St Petersburg train that killed at least 14 people.
Russian and Kyrgyz investigators said on Tuesday that Akbarzhon Jalilov, a 22-year-old Kyrgyz-born Russian national, was the prime suspect in the deadly bombing.
Neither country has specified whether the attack was a suicide bombing or if the bomber managed to escape.
"Genetic traces of Jalilov were found on a bag that contained an explosive device" that was left at a station near where the bomb went off, Russia's Investigative Committee said in a statement on its website.
There was no immediate claim of responsibility for the attack which came as President Vladimir Putin visited the city, Russia's second biggest and the president's hometown.
The Interfax news agency said authorities believe the suspect was linked to "radical Islamic groups" and carried the explosive device on to the train in a backpack.
Amateur video broadcast by Russian TV showed people lying on the platform of the Technological Institute station, and others bleeding and weeping just after the train pulled in with a huge hole ripped in the side of one of the carriages.
Within two hours of the blast, authorities had found and deactivated another bomb at another busy station, Vosstaniya Square, the anti-terror agency said. That station is a major transfer point for passengers on two lines and serves the railway station to Moscow.
Al Jazeera's Rory Challands, reporting from St Petersburg, said security services were investigating the suspect and the origin of the attack.
"They are looking at known groups and the prime suspect at the moment is likely to be ISIL as they have threatened Russia since its involvement in the Syrian conflict," he said.
"Russian authorities have been particularly worried that its citizens and those from Central Asia that have gone to Syria to fight alongside ISIL have brought their violence back home with them.
"The Syria connection is something which the Russians are quite sensitive about, and when [Russia's Foreign Minister] Sergey Lavrov was talking with his Kyrgyz counterpart, he said any attempt to link this attack with foreign policy in Syria would be construed as cynical at the very least."
Russia, a staunch ally of Syria's President Bashar al-Assad, has been bombing rebel-held areas since September 2015.
According to monitoring groups, Russian air strikes have killed more than 10,000 people, including nearly 4,000 civilians.
The city mourns
Residents laid flowers outside the city's subway as the three days of mourning began on Tuesday.
Patriach Kirill, head of the Russian Orthodox Church, led a service at Moscow's main cathedral for those killed in the blast.
"This terrorist act is a threat to all of us, all our nation," he was quoted as saying by the Interfax news agency.
In the past two decades, Russian trains and planes have been frequent targets of attack.
In October 2015, ISIL fighters downed a Russian airliner heading from an Egyptian resort to St Petersburg, killing all 224 people on board.
