ST. PETERSBURG, Russia — As part of a sweeping hunt for any accomplices of the St. Petersburg suicide bomber, investigators Wednesday rounded up seven suspected Islamic State recruiters from the Central Asia region of the former Soviet Union but found no immediate evidence of their involvement in the subway attack.

The Investigative Committee hasn’t caught any associates of 22-year-old Akbarzhon Dzhalilov, a native of the Central Asian nation of Kyrgyzstan. The committee, Russia’s top criminal investigation agency, said it’s looking into the possibility that Dzhalilov, who carried out Monday’s deadly attack, could have been linked to the militant group.

The impoverished, predominantly Muslim countries in Central Asia are seen as fertile ground for Islamic extremists, and thousands of their residents are believed to have joined IS in Syria and Iraq.

Meeting with the heads of security services from a regional alliance that includes most of Russia’s Central Asian neighbors, President Vladimir Putin warned that terror threats still loom over the region.

“We see that, unfortunately, the situation is not improving,” Putin said. “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.”

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.

The detainees were accused of seeking “mostly immigrants from the republics of Central Asia to commit crimes of a terrorist nature and encourage them to get involved in the activities of terrorist organizations,” the Investigative Committee said.