A court in Saint Petersburg said six suspects, aged 20 to 39, would be held until June 2 while the investigation into the attack continued.
They were arrested Thursday, and investigators said they found in one of the suspects' residences an explosive device "identical" to the one found at a metro station that was discovered shortly before another device exploded in a tunnel.
They also recovered firearms and ammunition from the suspects' residences, according to Russia's Investigative Committee. They face charges of terrorism and accessory to terrorism.
Two other suspected accomplices, a man and a woman, were arrested in Moscow on Thursday and have also been charged with terrorism, the TASS news agency said.
One of those, a 24-year-old, had admitted to being in contact with the suspected bomber, 22-year-old Akbarjon Djalilov, according to local press reports.
Djalilov's remains were found at the site of the blast and traces of his DNA were also discovered on a bag containing the bomb at another metro station that was successfully defused, investigators said.
They have not yet determined any motive for the attack, and are looking into possible links with the Islamic State group, though it has not claimed responsibility for the attack.
Security forces also arrested Wednesday in Saint Petersburg seven citizens from Central Asian countries who are suspected of recruiting "terrorists".
But the arrests are not linked to Djalilov "for the moment", the authorities said.
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