Investigators Explore Possible Link Between Boston Bombers, Extremist Group From Fox News: Investigators are exploring potential links between Boston Marathon bombing suspect Tamerlan Tsarnaev and an extremist group known as the Caucasus Emirate, sources tell Fox News. The purported leader of the separatist and Islamic extremist group is Doku Umarov, a Chechen Islamic militant in Russia whose ordering of numerous attacks on civilians has earned him the nickname Russia's Usama bin Laden. The U.N.'s Al Qaeda sanctions committee has also cited Umarov for his alleged connections with Al Qaeda and other extremist groups. The investigation is based on the contents of Tsarnaev's YouTube channel and his trip to Russia in the first half of 2012, which included stops in Dagestan and Chechnya. Sources tell Fox that videos deleted from his YouTube account include a “terrorist playlist” and links to Vilayat Dagestan, which is associated with Caucasus Emirate.