Seven people suspected of recruiting for so-called Islamic State have been arrested by police investigating the deadly explosion in St Petersburg.
Investigators rounded up the suspects, from the Central Asian region of the former Soviet Union, but found no immediate evidence of their involvement in the attack.
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.
The impoverished Muslim countries in Central Asia are viewed as fertile ground for Islamic extremists, and thousands from the area are believed to have joined IS in Syria and Iraq.
Meeting with heads of security services from a regional alliance which includes most of Russia's Central Asian neighbours, President Putin warned that the threat of terror still looms over the region.
Mr Putin said: "We see that, unfortunately, the situation is not improving.
"The recent tragic events in St Petersburg are the best confirmation of this. We know that each of our countries, practically every one, is a possible and potential target of terrorist attacks."