Reports emerge that the St. Petersburg metro blast, which killed 14 people, was the work of a suicide bomber, possibly from Kyrgyzstan.
The suspected bomber was identified as Kyrgyz-born Akbarjon Djalilov, a Russian citizen who was born in 1995.
In Wednesday's sweep in St. Petersburg, law enforcement agencies arrested seven Central Asian migrants who are suspected of acting as recruiters for the Islamic State and the al-Qaida's branch in Syria.
Before blowing himself up, Dzhalilov left a second bomb, hidden in a bag, at another subway station.
No one has yet claimed responsibility for the blast that tore through a subway carriage in Russia's second city on Monday.
Russian is still reeling after the Monday attack which took place on the day Russian President Vladimir Putin was visiting St Petersburg, his home city.
People clutching flowers and white balloons gathered at a rally outside Saint Petersburg's Technological Institute metro station, one of the two stations at the heart of the attack.
An explosive device, fire arms and ammunition have been found at homes of detained individuals, the Russian Investigative Committee said.
Syria chemical attack death toll now at 72; new strikes hit
The Organisation for the Prohibition of Chemical Weapons (OPCW) said it was "seriously concerned" by reports of Tuesday's attack. Speaking about Tuesday's gas attack , the White House condemned Assad but also lay blame on the Obama administration.
"Prime Minster expressed deepest condolences to the government and people of Russian Federation on the tragic loss of lives in the blast in St. Petersburg metro", said a PMO statement.
As well as those who died, dozens of people were injured in the bombing attack.
"Objects relevant to the investigation were found during the search of the apartment where these people lived", the Investigative Committee said.
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. Medyantseva was on the subway with one of her two daughters when the bomb went off.
People light candles during a religious service for victims of a subway explosion, in a cathedral in St.Petersburg, Russia, Wednesday, April 5, 2017.
She also said that an improvised explosive device (IED) was found in the detained suspects' apartments during a police raid, along with firearms and ammunition.
Residents have been evacuated and explosives experts have started working on the site.