Eight people suspected of involvement in the Saint Petersburg metro bombing were detained Thursday, Russia's Investigative Committee said.
"Six people were detained in Saint Petersburg and two in Moscow who are involved in the act of terror" following an investigation and search operation conducted with the FSB security service and interior ministry, the committee said in a statement.
Investigators said they discovered, in one of the suspects' residences, an explosive device "identical" to the one found at a Saint Petersburg metro station that was discovered shortly before another device exploded in a tunnel killing 13 people.
They also recovered firearms and ammunition from the suspects' residences, the committee said.
The committee said a court will soon rule on detention measures for the eight suspects, who seem to bear Central Asian names.
Investigators said earlier Thursday that they had raided the flat of "several citizens of Central Asian republics, who had been in contact" with the suspected bomber, 22-year-old Akbarjon Djalilov.
They confiscated other objects "relevant to the investigation" at his acquaintances' flat but did not disclose their nature.
Investigators said Djalilov -- believed to be a Russian citizen born in the Central Asian nation of Kyrgyzstan -- had set off a bomb in a train carriage travelling between two busy Saint Petersburg metro stations on Monday afternoon.
His 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.
No one has so far claimed responsibility for the attack.
The head of Russia's Investigative Committee Alexander Bastrykin has ordered officials to look into any potential "links" between the alleged attacker and the Islamic State group.
Jihadists from IS -- which includes foreign fighters from ex-Soviet Central Asia and the Caucasus region -- have repeatedly threatened an attack on Russian soil in revenge for Moscow's military backing of Syrian leader Bashar al-Assad.