The Interfax news agency says Russian police suspect that the explosion on a St Petersburg subway train was caused by a suicide bomber.
The agency quoted an unidentified law enforcement official saying that authorities had identified the suspected attacker on Monday was a 23-year old national of an ex-Soviet Central Asian nation. It didn't name the suspect or the country.
The bombing killed 11 people and wounded 45 others as Russian President Vladimir Putin was in St Petersburg, his home town.
Russian news reports had previously said that police were seeking a man caught on security cameras who was suspected of leaving a bomb behind him on a subway train.
Russian media published photos of the suspect wearing what appeared to be a skullcap characteristic of Russia's Muslim regions.
Interfax later quoted a law enforcement official saying that the man in the video had gone to police to profess his innocence.
City authorities shut the entire subway system after the blast. Law enforcement agents found an unexploded bomb at another subway station and defused it.
White House Press Secretary Sean Spicer said the US condemned the "reprehensible" attack, while US President Donald Trump called the deadly bomb blast on a Russian subway train "absolutely a terrible thing".
Interfax cited unnamed sources as saying the bomb, packed with shrapnel, may have been hidden in a train carriage inside a briefcase.
Russia has been the target in the past of numerous bomb attacks, frequently targeting public transport. Most are blamed on Islamist rebels from Russia's North Caucasus region. The rebellion there has been largely crushed, but security experts say Russia's military intervention in Syria has made Russia a potential target for Islamic State attacks.
The blast raised security fears beyond Russian frontiers. France, which has itself suffered a series of attacks, announced additional security measures in Paris.
Video from the scene of Monday's blast showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers.
A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
The Australian government urged travellers in the city to avoid affected areas and the metro, but did not change it's level of advice for the country.