Doku Umarov could be one of the most important -- and dangerous -- terrorists Americans have never heard of.
The Chechen Islamic radical, who leads a group called the Caucasus Emirate, for years has been one of Russia's most-wanted men. He and his organization have claimed responsibility for attacks killing dozens of civilians, including the 2010 Moscow subway bombings. Since 2011, the U.S. State Department has offered $5 million for information on his whereabouts.
Though nicknamed Russia's bin Laden, his name never had the global resonance of Al Qaeda's founder or its deputies. But in the wake of the Boston Marathon attack, Umarov is receiving worldwide attention.
Whether he or the Caucasus Emirate are in any way connected to the bombing is not known. Sources told Fox News that investigators are looking into possible ties between Tamerlan Tsarnaev, the suspect who died in a shootout early Friday in Boston, and the Caucasus Emirate.
A U.S. official said Tuesday that the "working theory" is they were motivated by radical Islam but acted alone. Still, officials have not determined what the older brother was doing during a mysterious six-month trip to Russia last year.
Though Umarov is not as well known in the U.S. as some other wanted terrorists, he is in the top tier. The 2011 world's most-wanted list, put out by Forbes, listed him along with the likes of Al Qaeda chief Ayman al-Zawahiri and brutal Ugandan warlord Joseph Kony.
The Chechnya-born militant has over the decades risen through the ranks to co-opt part of the Chechen separatist movement and twist it to fit his goals of establishing an Islamic emirate in the southern Russia region. If Chechen Islamic extremists are in fact making an attempt to extend their reach and plots beyond Russia's borders -- as some analysts have claimed -- he would play a central role in that.
According to a 2011 report by the Center for Strategic and International Studies, Umarov has since 2007 declared "jihad against anyone fighting against Muslims anywhere across the globe."
The Caucasus Emirate, the extremist group he leads, was formed in 2007. It grew out of the flagging separatist group the Republic of Ichkeria, which Umarov helped lead in various positions since the first Russian-Chechen war. He climbed the ladder until creating the Caucasus Emirate and declaring himself emir.
Umarov was already wanted in Russia -- the country charged him with murder in 1992, and an international arrest warrant was issued in 2000 -- before the formation of the CE, but his group's crimes have since attracted more global attention.
He claimed responsibility for the 2010 Moscow bombing of two subway stations, which killed 40, and the 2011 attack on a Moscow airport which killed nearly as many.
After these attacks, a United Nations committee formally listed him in 2011 as being associated with Al Qaeda, Usama bin Laden or the Taliban. The U.N. declared that he was commanding groups in the North Caucasus, "and organizes major terrorist acts and coordinates the provision of resources to militants." The U.N. tied him to several Islamic extremist groups.
Shortly afterward, the U.S. State Department offered a $5 million reward for information leading to his location.
The State Department noted his group had launched terror attacks using improvised-explosive devices (IEDs), and that the group's declared enemies include "the United States as well as Israel, Russia and the United Kingdom." The department listed Umarov as a "specially designated global terrorist" a year earlier.
Umarov and his group compose only part of the Chechnya separatist movement, and his group is not part of the formal Chechen regional government.
Even within the CE, he has been controversial. His temporary resignation in 2010 led to a split, and he eventually reasserted control over the group.
According to the CSIS, he appoints the lower-level emirs, who then pledge loyalty to him. The group's targets and goals have changed over time. The CSIS report said suicide bombings started within weeks after he revived a bombing unit called the Riyadus Salikhin Martyrs' Brigade.
But then in 2012, he ordered that attacks on Russian civilians be stopped.
Following reporting by Fox News that the CE was being looked at in the wake of the Boston attack, the group put out a statement that appeared to distance itself from the attack.
"After the events in Boston, the US, information has been distributed in the press saying that one of the Tsarnaev brothers spent 6 months in Dagestan in 2012. On this basis, there are speculative assumptions that he may have been associated with the Mujahideen of the Caucasus Emirate, in particular with the Mujahideen of Dagestan," the group said. The organization said it is "not fighting against the United States of America. We are at war with Russia, which is not only responsible for the occupation of the Caucasus, but also for heinous crimes against Muslims."