Eight people are being held in connection with the bombing on St Petersburg metro, Russian investigators have confirmed.
The arrests - six in Moscow, two in St Petersburg - came three days after 13 people were killed in the attack.
Earlier on Thursday, an explosive device was made safe in a flat in St Petersburg by Russian police.
Neighbours were moved away and witnesses told local media they saw men led out in handcuffs.
The main suspect in the metro bombing, Akbarzhon Jalilov, also died in the explosion.
He was aged 22 and from the Central Asian republic of Kyrgyzstan. His remains were identified by his parents on Wednesday, and by DNA testing.
Russian investigators are examining tape, tin foil and some other suspicious items found at Jalilov's St Petersburg flat.
Russia's Investigative Committee (SK) says they appear similar to components found in a device left at Ploshchad Vosstaniya metro station on the day of the bombing.
The flat raided at around 05:00 local time (02:00 GMT) on Thursday was in Tovarishchesky Prospekt in the east of St Petersburg.
"An explosive device found in the flat has been made safe. Several suspects have been arrested; they didn't resist and there's now no threat to local people," the head of the local authority Konstantin Serov was quoted as saying.
Sources told Interfax news agency that investigators were examining possible links between the men and the alleged bomber.
Russia's Investigative Committee (SK) said on Thursday they had established that "several citizens of Central Asian republics were in contact with Jalilov".
Eight people from Central Asia were also arrested in the city on Wednesday as part of the metro bomb investigation. The SK said they were held for allegedly recruiting for Islamist militant groups such as so-called Islamic State and Jabhat al-Nusra since 2015, and "committing crimes of a terrorist nature".
The SK has named the 13 people who died, aside from Jalilov, on Monday afternoon after the train had left Sennaya Ploshchad station. About 50 injured are being treated in hospital.
The dead were named as:
- Irina Medyantseva, 50, who died as she tried to shield her daughter from the explosion, according to reports in Russia. Her daughter Yelena, 29, was treated for her injuries in hospital and her condition was said to be stable
- Dilbara Alieva, 20, from Azerbaijan, who was taken to hospital but later died from her injuries
- Maxim Aryshev, a 20-year-old Kazakh citizen and student at St Petersburg state university
- Yury Nalimov, 71
- Ksenia Malyukova, 18
- Angelina Svistunova, 27
- Oksana Danilenko, 25
- Larisa Shchekina, 66
- Denis Petrov, 25
- Mansur Sagadeev, 16 or 17
- Dmitry Mazanov, 27
- Yulia Krasikova, 25
- Maria Nevmerzhitskaya (53)