Boston bombings investigators following new leads in Russia

BOSTON — U.S. officials investigating the Boston Marathon bombings are following new leads in Russia.

They are trying to piece together how one of the suspects may have become radicalized during a trip there last year.

The Russian Mosque, a known gathering spot for extremists, is the new focus of the investigation.

It is there U.S. officials believe that Tamerlan Tsarnaev met with Mansur Nidal, who is a known Chechen terror recruiter killed last year by Russian authorities.

In new video, obtained by "Entertainment Tonight," for the first time Tamerlan's voice can be heard.

The young American boxer, who authorities now say had Internet contact with William Plotnikov, a Canadian man who authorities said moved to Russia and joined a terror group.

The alleged connections offer new clues about what may have inspired Tsarnaev to terror during the trip to Russia.

U.S. intelligence officials are now investigating how warning signs may have been missed.

"We want to leave no stone unturned. We want to see, is there in fact additional protocols and procedures that could be put in place? We won't know that until that review is completed," President Barack Obama said Tuesday.

Meanwhile, the hunt for possible accomplices continues.

ABC News has learned authorities have identified almost a dozen persons of interest, including Tamerlan's widow, Katherine Russell. 

Russell has submitted DNA samples to the FBI.

WSOC