Quote :
Tamerlan left the United States in January 2012 for Russia and returned in mid-July, according to records uncovered by NBC in New York.

But an official inside the Department of Homeland Security with knowledge of federal law enforcement activities in Massachusetts has claimed to MailOnline that Tamerlan was on the radar screen of agents in Boston between his return to the U.S. and the end of the fall.

The source, who has been contradicted by other officials, said that in 2012 federal law enforcement received tips from inside a Boston mosque about young Muslims, including some converts, who were becoming radicalized into anti-American zealots as their knowledge of their religion grew.

Tamerlan was allegedly one of those radicals, and attracted the attention of an informant working with an agency attached to the Boston-area Joint Terrorism Task Force (JTTF). That informant communicated observations about the man to a government handler, the source said.  

http://www.dailymail.co.uk/news/article-2311909/Boston-news-Feds-investigating-possibility-governments-terror-trackers-knew-Boston-bombers-blasts.html