According to a source close to the Russian security services who specializes in religious radicalism, Tsarnaev attended services at the mosque on Kotrova Street during both of the extended visits he made to Dagestan over the past two years. That is why Russia’s Federal Security Service, the agency better known as the FSB, sent a warning to the FBI in 2011 to be aware of Tsarnaev’s possible links to extremism.

In a statement on April 19, the FBI said it had received information from an unidentified “foreign government” that Tsarnaev was “a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared … to join unspecified underground groups.” In response, the FBI said it interviewed Tsarnaev and checked its records for relevant information, but “did not find any Russia’s counterterrorism activity, domestic or foreign.”

According to the source in the regional capital of Makhachkala, who spoke to TIME on Monday, Tsarnaev was monitored by Russian counterterrorism forces for at least one month in 2011 and throughout his six-month stay in Dagestan last year. “There wasn’t enough time [in 2011] to come to any conclusions about the extent of his involvement [in Islamist extremism],” the source says, asking to remain anonymous due to the sensitivity of the matter. “So we asked our American colleagues to follow up.”