Russian security agents have defused an explosive device found during a search of an apartment in St Petersburg following the city's train bombing.
They have also made three arrests in connection with Monday's attack.
News agency Interfax, citing a law enforcement source, said alleged links with the bomber, who also died in the explosion, were being checked.
Security services in Kyrgyzstan have named the suspect as Akbarzhon Jalilov, who was born in the country's second city of Osh and had Russian citizenship.
He detonated the device on a busy metro line, killing himself and 13 others as well as wounding more than 50 people.
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Jalilov is believed to have links to radical Islamist groups, says Interfax.
A spokesman for Russian president Vladimir Putin would not comment on the reports, saying it was up to law enforcement agencies to provide details about the investigation.
The bomb was detonated on a train between Sennaya Square and Technological Institute, two busy stations in the centre of the city.
A second explosive filled with shrapnel and more than three times the size of the first was found and made safe later in the day at the Vosstaniya Square station, which also serves the rail line to Moscow.
On Thursday, the Federal Security Service said its experts defused a self-made explosive device found during a search carried out in connection with the bombing.
The discovery and arrests of suspected acquaintances came after police in St Petersburg detained several people over "Islamic State recruitment" earlier this week.
Authorities say there is no evidence those suspects, from former Soviet Central Asia, are connected to the bomb attack.
However, they are suspected of actively recruiting other Central Asian migrants to join Islamic State and another militant group, the Nusra Front, an investigative committee said.
IS, which has fighters from former Soviet Central Asia among its ranks, has repeatedly threatened to attack Russia in revenge for Moscow's backing of Syrian president Bashar al Assad.
Investigator Svetlana Petrenko said the driver of the train saved lives by continuing on to the next stop, reducing the danger to passengers who would have had to walk along electrified tracks.