Eight members of extremist cells have been arrested in connection with last week's deadly bombing on the subway in St. Petersburg, Russia's intelligence chief said Tuesday.
The suicide bombing in Russia's second largest city killed 13 passengers and injured dozens of others.
Akbardzhon Dzhalilov, a 22-year old Kyrgyz-born Russian national, has been identified as the bomber. Russian authorities have not outlined his possible links to extremist groups but an unidentified law enforcement official told the Tass news agency that investigators were checking information that he may have trained with the Islamic State group in Syria.
Tass said he reportedly flew to Turkey in November 2015 and spent a long time abroad.
No one has claimed responsibility for the bombing, but Russian trains and planes long have been targeted in bombings by Islamist militants.
Alexander Bortnikov, chief of the FSB, the main successor to the KGB intelligence agency, said in comments carried by Russian news agencies on Tuesday that six members of terror cells were detained in St. Petersburg and two in Moscow in connection with the attack. Bortnikov said all of them hail from former Soviet Central Asian republics and that the police found a large amount of weapons and ammunition at their homes.
Bortnikov admitted intelligence agencies failures leading up to the attack.
"The investigation in the St. Petersburg subway attack showed that the operative work did not fully meet the threat from terrorist organizations," he was quoted as saying.
Russian-based extremist groups are mostly made up of migrant workers who come from Central Asia and recruit in the migrant community, Bortnikov said, calling for tighter restrictions on immigration.
The impoverished, predominantly Muslim countries in Central Asia are seen as fertile ground for Islamic extremists, and thousands of their citizens are believed to have joined the Islamic State group in Syria and Iraq.
Russian President Vladimir Putin has said between 5,000 and 7,000 people from Russia and other former Soviet republics were fighting alongside Islamic State and other militants in Syria.