"U.S. law-enforcement officials said the two brothers came to the U.S. at different times?one with his parents in 2002 and the other on his own in 2004. Intelligence and counterterrorism officials have been scouring records to look for intersections with the two men, officials said. Records indicate one or both of the bombing suspects had spent time in the Caucasus region since they first immigrated to the U.S. and then returned to the U.S., officials said.
The brothers' father lives in the republic of Dagestan"
"Over in the North Caucausus, meanwhile, the Kremlin is carrying out a brutal and rolling counter-insurgency campaign. Its focus is Dagestan, the neighbouring state to Chechnya, and now a hotbed of violent jihadist rebellion. Three weeks ago, Umarov ['self-proclaimed Emir of the Russian North Caucasus'] appealed to Chechen fighters abroad to come home to take part in the fight"
Both from guardian.co.uk