Suspected accomplices of the man believed to have carried out the St Petersburg metro bombing have been arrested, according to Russian news agency Interfax.
Links between those arrested and the bomber are still being investigated, the agency said, but added that "clues" had been found at the suspected accomplices' addresses.
Monday's explosion killed 14 people, including the bomber, and injured 55 others.
Kyrgyzstan-born Akbarzhon Dzhalilov, 22, has been identified as the man behind the attack.
Seven suspected IS recruiters have been arrested by authorities investigating the explosion, but officials found no immediate evidence of their involvement in the attack.
The seven people were from Central Asian countries and investigators say they were recruiting for IS.Read the full story ›
A St Petersburg underground station closed after a bomb threat on Tuesday has reopened, Russian news agency RIA reported.
Sennaya Ploshchad station was shut earlier after an anonymous call warning of a bomb.
Monday's deadly blast happened on a train as it travelled between Sennaya Ploshchad and Tekhnologichesky Institut underground stations.
One of the underground stations affected by Monday's bombing in St Petersburg has been closed after a bomb threat, Russian emergency officials have said.
The agency said Sennaya Ploshchad station had been shut after an anonymous call warning of a bomb.
The blast on Monday happened on a metro train as it travelled between Sennaya Ploshchad and Tekhnologichesky Institut underground stations, killing 14 people.
The death toll from the St Petersburg metro bombing has risen to 14, Russia's health minister has said.
Veronika Skvortsova said 11 people died at the scene, one died in an ambulance and two in hospital.
She added that 49 people are still in hospital.
Theresa May has condemned the "horrific" bombing in St Petersburg which killed 11 people on Monday.
The prime minister said she had written to Russian President Vladimir Putin to express her condolences for the "absolutely appalling attack".
"Our thoughts must be with the victims and their friends and families," she said.
"This shows the terrible terrorist threat we are all facing.
"I'm very clear, as the people of the UK were after the attack in London, that we will prevail, the terrorists will not win, our values will prevail."
The St Petersburg subway bomber has been identified as a Kyrgyzstan-born Russian citizen, according to Kyrgyz security services.
The Kyrgyz intelligence agency said the suspect was born in Osh, the country's second largest city, in 1995.
Kazakhstan's security services said it was working with Russia to investigate Monday's blast, which killed 11 people.
The St Petersburg metro explosion has been condemned as a "barbaric and cowardly terrorist attack" by the UN Security Council.
Offering condolences to the victims of the blast - which happened between Sennaya Ploshchad and Tekhnologichesky Institut underground stations - council members released a statement saying the "perpetrators, organizers, financiers and sponsors of these reprehensible acts" should be brought to justice.
Donald Trump has offered his condolences to the victims of the St Petersburg metro explosion.
According to the White House the US President called his Russian counterpart Vladimir Putin on Monday to offer "the full support of the United States government in responding to the attack and bringing those responsible to justice."
Russia President Vladimir Putin has laid a floral tribute at a impromptu memorial to the victims of a bomb blast on the St Petersburg metro station.
Mr Putin happened to be in Russia's second-largest city for a meeting with his Belarusian counterpart Alexander Lukashenko when the explosion took place.