As the agency was accused of "dropping the ball" over the case, NBC News reported that Tamerlan Tsarnaev had been seen making six visits to a known Islamic militant in a mosque in the Russian republic of Dagestan.
The visits came during a six month trip that Tamerlan made to the city of Makhachkala to see his family, NBC said.
According to a local police official, a case file on Tsarnaev was then handed over to the FBI along with a request for further information. However, the FBI never replied.
The agency has already admitted that it interviewed Tamerlan Tsarnaev in 2011 after Russia raised concerns that he was becoming a follower of radical Islam, but found nothing "derogatory" against him and did not pursue the case further.