Bomb disposal experts made the explosive device found at the apartment building safe after evacuating people living in flats on two stairwells.
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. A bomb blast tore through a subway train under Russia's second-largest city Monday, killing several people and wounding more.
The Monday explosion claimed the lives of at least 14 people, 55 others remain in hospitals.
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.
Serov said the people inside the apartment where the explosive device was discovered were not the owners of the residence. Thousands of residents of these countries, which sprang from the ruins of the former Soviet Union, are believed to have joined the Islamic State in its strongholds in Iraq and Syria.
Russia's Investigative Committee has not caught any of 22-year-old Akbarzhon Dzhalilov's associates, but is looking into the possibility that he could have links with the extremist group. He has named Daesh as one of the reasons behind Russia's military campaign in Syria in support of Syrian President Bashar Assad.
With St. Petersburg residents tense following the deadly subway bombing, the collapse of part of apartment building on Thursday triggered fears of a new attack until city authorities explained the cause. Earlier on Thursday, mourning continued in St Petersburg as the jittery city laid to rest the 14 people killed in the attack.
The detainees are suspected of being active since November 2015 in recruiting other Central Asian immigrants for "terrorist activity", and for ISIS and another banned Islamist movement, Fateh al-Sham, previously known as the Nusra Front.
Neighbours were moved away and witnesses said three men were led out in handcuffs, the agency said. Her husband, Alexander Kaminskiy thanked everyone saying "The country and the entire world share our pain".
Cubs' Baez collides with Heyward year after Schwarber injury
This is a game I see Chicago bouncing back and running away with, so I'll take them on the run line to give my bet more value. Baez gave up a thumbs up and was able to walk off after the play, but the Cubs understandably removed him from the game.
Kevin Durant practices, set to play Saturday in return
I didn't look at it as a career-threatening injury. "If I had to wait until the playoffs, I would have waited", he said. He did wear a brace and says he has not decided whether to wear one Saturday night. "I didn't know what was going on".