Russia's security chief on Tuesday said all those detained over the Saint Petersburg metro bombing originate from Central Asia, urging tighter migration controls.
The eight people detained in Moscow and Saint Petersburg over the April 3 bombing that killed 13 are "all originally from Central Asia", the head of the FSB security service Alexander Bortnikov said, quoted by RIA Novosti news agency.
Bortnikov, speaking at a meeting of the National Antiterrorism Committee (NAK) that he chairs, did not specify the suspects' current citizenship.
"To prevent attempts by fighters to enter Russia," there is a need for "additional measures of control on the state border... towards those suspected of taking part in terrorist attacks," the security chief said.
The country's border guards are under the control of the FSB.
Russia's Investigative Committee last week named the eight suspects but not their ethnic origin.
The suspected suicide bomber has been named as 22-year-old Akbarjon Djalilov, who is thought to be a Russian national born in Central Asian Kyrgyzstan.
Millions of migrants from the poverty-stricken ex-Soviet countries of Uzbekistan, Tajikistan and Kyrgyzstan come to work in Russia.
The core of "terrorist groups in Russia" is made up of citizens of ex-Soviet countries arriving "in flows of migrant workers," Bortnikov said.
Some of these had trained and fought in Syria and Iraq, the FSB chief added.
President Vladimir Putin has repeatedly warned that Russia faces a threat from hardened Islamist fighters returning from Syria.
The investigation into the Saint Petersburg metro bombing shows that "operational work on the ground still does not fully correspond to the threat from illegal terrorist organisations," Bortnikov said.
Bortnikov linked the Saint Petersburg attack to the Islamic State jihadist group, which has however not claimed responsibility.
He cited information according to which "the heads of terrorist groups active in the Middle East as part of IS are working on plans to carry out terrorist attacks in various regions of the world, including Russia."
"Examples of this are the high-profile terrorist attacks in recent weeks in Britain, Russia, Sweden and Egypt," Bortnikov said.
In Sweden, an Uzbek man is suspected of driving a truck into pedestrians in the capital Stockholm, killing four and injuring 15.
IS has not claimed responsibility, but Swedish media reported the suspect had told investigators that he had received an "order" from the group.