Russian authorities detained 140 suspected Muslim extremists in the capital city of Moscow on Friday. At least 30 of those now in custody are foreign nationals.

Russian Federal Security Service officials told RT that the suspects were detained in a house of worship where visitors often “converted to radicalism and joined militant groups active in the North Caucasus, as well as participated in preparing and perpetrating terrorist acts in Russia.”

Most Russian encounters with extremist violence have occurred in the North Caucasus, primarily in the Muslim-majority federal subjects of Chechnya and Dagestan, where insurgent groups have long sought greater autonomy.

The region became an epicenter of international focus this month after brothers Dzhokhar Tsarnaev and Tamerlan Tsarnaev, who are ethnic Chechens, set off two deadly bombs at the finish line of the Boston Marathon on April 15, killing three and injuring 280.

Russian and American authorities agree that the Boston perpetrators acted alone and weren't directly connected to any Islamist groups in the North Caucasus.

Friday’s detentions in Moscow recall arrests made in Russia in 2011, when four people from the North Caucasus were detained on charges of plotting bomb attacks in the capital city. Those suspects attended the same prayer house where the 140 alleged extremists were arrested Friday. Russian security forces remain vigilant, but anti-extremist laws in Russia have been criticized by human rights groups around the world for their vague wording, which has allowed security forces to monitor and detain people who have committed no crimes. Muslims, Christians and even secular political dissidents have been detained by authorities on suspicions of extremism, incitement and even hooliganism in recent years.