Three Syrian nationals were arrested on Sunday in Moscow on suspicion of plotting a terrorist attack, Moskovsky Komsomolets reported Monday. The arrests were made in response to an alert from a neighbor.

According to a source inside Russia’s National Counter-Terrorist Committee, security forces have captured a group of 15 people in downtown Moscow. An improvised explosive device (IED) was discovered and defused in the suspects’ apartment.

Russian authorities believe the terrorist cell planned to blow up the explosive device in a metro station or in one of Moscow’s airports.

12 suspects are from around Chechnya, and three are Syrian citizens, according to the same source. Earlier reports listed only three detainees.

Reports say more than 100 residents in the building where the terrorist cell had been hiding were evacuated before the raid, and gas lines were cut off.

Russia has been an ally of Syria since 1956, and has supported the Syrian government with military aid in the form of weaponry, training, and military advisors since the Civil War began in 2011. In September 2015, the Russian Federation Council approved the use of the Russian military in Syria to fight terrorist groups and ISIL, at the request of the Syrian President Bashar Assad. Despite the fact that Russian officials have been stating that Russian strikes are targeting only ISIL, Syrian rebel groups, including al-Nusra, al-Qaeda’s Syrian branch, are reporting also being hit by Russian raids.