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.

Investigators were still checking their contacts, it added, but noted that as of now, “there is no information about any links between the detainees and the perpetrator of the terror attack.”

The investigators also have searched Dzhalilov’s home and found objects similar to those used in the subway bomb, it said.

CCTV footage from outside his apartment building showed him leave home with a bag and a backpack on Monday.

The explosion on a train running along a busy north-south subway line killed the attacker and 13 other people. Another 55 have remained hospitalized, and several of them are in critical condition, according to the Russian Health Ministry.

Before blowing himself up, Dzhalilov left a second bomb, hidden in a bag, at another subway station. Police found and deactivated it, and Dzhalilov’s DNA was found on the bag.

At Dzhalilov’s hometown of Osh in southern Kyrgyzstan, his schoolteacher described him as “well brought up, calm and balanced.”

Fatima Saipjanova, who taught Dzhalilov at School-Gymnasium No. 26, said she never saw him get into any trouble. “I do not believe that this boy could do something wrong,” she said.

Like many others from Central Asia, Dzhalilov moved to St. Petersburg with his parents and eventually got Russian citizenship. He worked at a car repair shop and a sushi bar, and stayed in the city when his family returned home.

His parents arrived Wednesday in St. Petersburg for questioning after being interrogated by Kyrgyz security services. They identified his remains, according to the Investigative Committee.

Churches across the city held prayers for victims killed in the attack, including Irina Medyantseva, a 50-year-old artist who was locally famous for the dolls she made and sold. Medyantseva was on the subway with one of her two daughters when the bomb went off. She apparently shielded her 28-year-old daughter, Alyona, who survived.

“Irina was a very fun, creative person, she was full of plans,” Medyantseva’s sister, Anna, recalled. “She was a very good mom. She did a lot for her children.”

Anna attended one of the services, at St. Trinity Church, holding red tulips and a picture of her sister with a black ribbon tied to it.

After the attack, several politicians have called for restoring capital punishment in Russia. But parliament speaker Vyacheslav Volodin dismissed the statements and upbraided the politicians, saying: “One must not use a tragedy to promote oneself.”

Isachenkov reported from Moscow. Jim Heintz in Moscow contributed.