ST. PETERSBURG, Russia -- Russian investigators on Thursday arrested several people suspected of links to a suicide bomber accused of attacking the city’s subway earlier this week, and found elements of an explosive device in the apartment where they lived.

The Investigative Committee said that the suspects are residents of the former Soviet Central Asia region like the bomber, 22-year-old Akbarzhon Dzhalilov, a native of Kyrgyzstan.

Hours later, a large explosion was heard from the same building where the arrests were made. Russian offcials said nobody was hurt in the blast, but it wasn’t immediately clear whether it was a controlled explosion carried out by bomb technicians.

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 the Islamic State of Iraq and Syria (ISIS).

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The committee, the nation’s top criminal investigation agency, said investigators also found objects that would help advance the probe during a search of the home on St. Petersburg’s eastern outskirts.

Russian news agencies reported that law enforcement agencies arrested three people.

Russian police officer with a sniffer dog crosses a police line in St. Petersburg, April 6, 2017, amid arrests following a bomb attack on the city’s subway system. AP

The building’s residents had been evacuated before explosives experts went in to check the site. Konstantin Serov, the chief administrator in the city district where the building is located, told Russian news agencies that police deactivated an explosive device at the apartment.

Police in the city are on high alert following Monday’s explosion that killed the attacker and 13 other people and wounded some 55.

Thursday’s arrest came a day after law enforcement agencies detained eight Central Asian migrants suspected of acting as recruiters for ISIS and al Qaeda’s Syria branch. The investigators found no immediate evidence of their involvement in the subway attack.

No one has claimed responsibility for the subway bombing, but Russian trains and planes long have been targeted by bombings staged by Islamist militants.