Russia’s Metro Bombing Suspect ' From Kyrgyzstan'
At least 14 persons were killed while 49 others were injured in the explosion between two underground stations on Monday.
The Kyrgyz security service named the bomber as Akbarzhon Jalilov, who it said was born in Osh in 1995.
There are conflicting reports as to whether he was a suicide bomber.
The BBC quoted Kyrgyz state media as saying that the country's security service is "maintaining contact with the Russian secret service for further investigation".
Authorities in St Petersburg have declared three days of mourning.
The death toll rose from 11 to 14 on Tuesday after three people died in hospital.
Russian President Vladimir Putin, who was in St Petersburg - his home city - when the blast occurred, visited the scene on Monday and laid flowers at a makeshift shrine.
Russian investigators have given few details. No group has claimed responsibility.
Senior investigator Svetlana Petrenko told Russian media the train driver's decision to continue to the next station almost certainly helped save lives, as it allowed people to be rescued quickly.
Andrei Przhezdomsky, the head of the National Anti-Terrorist Committee, said the explosion was caused by "an unidentified explosive device".
The discovery of an explosive device at another station, Ploshchad Vosstaniya, also suggested a co-ordinated attack.
Interfax said the bomber was known to have links to radical Islam.
The news agency said he died in a suicide blast and was identified through his remains. But the president's spokesman, Dmitry Peskov, would not comment on reports a suicide bomber was to blame.
Another agency, Tass, reported that a woman may also have been involved in the explosion.
St Petersburg's metro system, which is used by more than two million passengers every day, has not suffered previous attacks.