8 Arrested in Connection with St. Petersburg Bombing

The suicide bombing on the subway in Russia's second largest city killed 13 passengers and injured dozens, AP reported.

Akbardzhon Dzhalilov, a 22-year old Kyrgyz-born Russian national, has been identified as the bomber. Russian authorities have not reported his possible links to extremist groups but an unidentified law enforcement official told the Tass news agency that investigators were checking information that Dzhalilov may have trained with ISIS in Syria. Tass said he reportedly flew to Turkey in November 2015 and spent a long time abroad.

No one has claimed responsibility for Monday's subway bombing.

Alexander Bortnikov, chief of the FSB, the main successor to the KGB intelligence agency, said in comments carried by Russian news agencies 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 that intelligence agencies failed.

"The investigation in the St. Petersburg subway attack showed that the operative work did not fully meet the threat from terrorist organizations," Bortnikov 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 extremists, and thousands of their citizens are believed to have joined ISIS (also known as Daesh).

Russian President Vladimir Putin has said between 5,000 and 7,000 people from Russia and other former Soviet republics were fighting alongside ISIS and other militants in Syria.