Tamerlan Tsarnaev interviewed by FBI two years prior to Boston Marathon attack

Federal law enforcement officials are reportedly investigating the possibility that Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev were known to the government’s terror-trackers.

Any inquiry into Boston Marathon bombing will look into whether agents could have taken action – but didn’t – to prevent the Tsarnaev brothers from obtaining the explosives that killed 3 and wounded 173 others.

CBS News reported Friday evening that two years ago, the FBI interviewed Tamerlan Tsarnaev, 26, who was killed in Thursday night’s dramatic shootout. The feds reportedly spoke to him at the request of an unspecified foreign government, but couldn’t establish that he had ties to terrorist radicals.

Tamerlan Tsarnaev left the US in January 2012 for Russia and returned in mid-July, according to records uncovered by NBC in New York.

An official inside the Department of Homeland Security with knowledge of federal law enforcement activities in Massachusetts has claimed that Tamerlan Tsarnaev 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 Tsarnaev 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.

A second source, formerly assigned to a US JTTF, confirmed that “there is some very quiet discussion in the Boston JTTF about this”.

The formerly JTTF-affiliated agent said that right now: “These guys are more interested in catching the younger terrorist alive, and interrogating him, than in worrying about what the different agencies knew and when they knew it.”

The Homeland Security claimed that the FBI in Boston now believes Tamerlan Tsarnaev was receiving assistance from an “organized radical element” in Massachusetts. And agents are now reviewing at least one report from inside a mosque to see if they can identify Tamerlan Tsarnaev’s associates in Boston’s Muslim community.

Law enforcement at the federal level has “practically nothing” on Dzhokhar Tsarnaev, Tamerlan’s younger brother.

The Associated Press reported late Friday evening that Dzhokhar  Tsarnaev was in police custody.

Both sources spoke on condition of anonymity because they are not permitted to talk about ongoing investigations.

The source also claimed that the agency said Tamerlan Tsarnaev turned up in a tip from an “undercover asset”.

Ultimately Tamerlan Tsarnaev was allegedly deemed “a low priority” and federal law enforcement “did not designate them [Tsarnaev brothers] as suspects or persons of interest in any crime”.

The Tsarnaev brothers also were not flagged for deportation by US Immigration and Customs Enforcement.

They immigrated to the US and were granted asylum in 2002.

Dzhokhar Tsarnaev, now 19, received his green card two years later. He became a naturalized US citizen on September 11, 2012.