Russian investigators on Thursday carried out raids on acquaintances of the suspected St Petersburg metro bomber and made several arrests in the Russian port city.
During the raids, explosives were also found which authorities say are similar to the unexploded bomb found in the city's metro after Monday's suicide bombing.
"Explosives were in a similar amount that was used at Vosstaniya (metro station)," a security source said.
After evacuating residents from the residential building, the device was defused.
One resident, who gave his name only as Anatoly, said that he had seen police detain four young men occupying an eighth floor apartment next to his own.
Police obtained evidence which will now be sent for analysis.
"Objects relevant to the investigation were found during the search of the apartment where these people lived," the investigative committee probing the bombing said.
"They were all confiscated and sent for analysis."
St Petersburg is still reeling after a suicide bomber detonated his explosives inside a train on the city's metro killing 14 people and injuring 50.
Authorities said those targeted and arrested in Thursday's search were "several citizens of central Asian republics, who had been in contact" with suspected bomber 22-year-old, Kyrgyz-born Russian citizen Akbarjon Djalilov.
Authorities said on Wednesday that they had searched Jalilov's residence and said CCTV footage showed him leaving his home ahead of the attack "with a bag and rucksack."
The attack has put renewed focus on the large number of migrants from central Asian former Soviet states, who have moved to Russia to work.
Russia's state investigative committee, a body with sweeping powers that is looking into the bomber's background, said in a statement it was looking into the backgrounds of people it suspected of being accomplices.
Meanwhile, an officially sanctioned solidarity rally with St Petersburg and concert is set to be held on Thursday evening by the Kremlin walls in the capital Moscow. Earlier on Thursday, mourning continued in St Petersburg as the jittery city laid to rest the 14 people killed in the attack.