The Investigative Committee, the state body leading the investigation, said footage from security cameras near the same building showed the suspect, Akbarzhon Jalilov, leaving home carrying a bag and a rucksack. It did not specify if that was on the day of the attack.
Jalilov was born in 1995 in Kyrgyzstan, an ex-Soviet Republic in central Asia, and held a Russian passport.
The apartment Jalilov rented in St. Petersburg is about 20 kilometers from the site of the explosion, in the center of the city, Reuters reported.
Neighbors at the building said that Jalilov had first moved into apartment 109 around a month ago.
In the several years Jalilov spent living in Russia, he blended into the millions of migrants from central Asia, and led an outwardly secular lifestyle. His profile on VKontakte, the Russian equivalent of Facebook, showed he was interested in pop music, fast cars, and boxer Mike Tyson.
Six people of central Asian origin have been held on suspicion of recruiting for radical groups, but there is no proof linking the detainees to the metro bombing, Russian investigators said.
Meanwhile authorities have beefed up security across major cities, with sniffer dogs and bag checks at several metro stations in Moscow.