Among the more unusual aspects of what has been learned thus far about the Brothers Tsarnaev is that in January 2011 Russian officials encouraged their U.S. counterparts to take notice of Tamerlan, the older of the two, for possible Chechen terrorist links. The only known result of the interviews that followed was to delay processing of Tamerlan's U.S. citizenship application. (His younger brother, Dzhokhar, became a citizen on Sept. 11, 2012.) But the Russian tip was part of the process that led to a subsequent agreement between that country and the U.S. concerning Chechen terrorists. The May 26, 2011, agreement -- the Joint Statement of the Presidents of the United States of America and the Russian Federation on Counter-terrorism Cooperation -- can be found on the White House website.

The key passage from that agreement with respect to the events in Boston is this: "We reaffirmed our common view of the threat to global security posed by Al Qaeda and advocated continued cooperation to disrupt, dismantle, and defeat Al Qaeda. Identifying and isolating terrorists is integral to our efforts. We welcomed Russia's efforts to update and enhance the implementation of the sanctions regime under United Nations Security Council Resolution 1822, and the decision by the United States to list Doku Umarov of the so-called 'Caucasus Emirate' as a specially designated global terrorist under U.S. Executive Order 13244."

For Westerners, Doku Umarov and the Caucasus Emirate may be among the least familiar elements of the legions of radical jihadist groups that thrive in the Muslim world from the Caucasus region of Russia, including the separatist Chechen Republic, through Afghanistan and Pakistan, and across the Middle East. In October 2007, Umarov declared himself emir of an emirate that stretches from the Black Sea in the west to the Caspian Sea in the east. He justified it as something he was required to do as a Muslim following Sharia law. In the same declaration, Umarov described the U.S., Britain and Israel as enemies of Muslims everywhere. Based on these facts, Umarov and the Caucasus Emirate appear to share radical jihadist characteristics routinely found in more familiar groups including Hezbollah in Lebanon and the Muslim Brotherhood in Egypt.

Independent national security consultant John Roberts was struck by the timing of the Russian warning to the U.S. about Tamerlan Tsarnaev in 2011 and the trip Tsarnnaev reportedly made to Russia later that year. "Because it takes about two years to mount insider terrorist attacks [identifying people to recruit, recruiting them, training them, selecting and planning the target, etc.] the Boston Marathon bombings make sense as a reprisal for our cooperation with the Russians against the Caucasus Emirate," Roberts told The Washington Examiner. He also noted that "we should have known after signing the May 26 agreement that it would mean the Chechens would start to target us co-equally with the Russians. Somebody dropped the ball, failed to do the math." All of this is informed speculation at this point, but law enforcement agencies should quickly determine whether the U.S. needs to be on high alert against Chechen terrorism.