One became an American citizen last year on Sept. 11. The other was a boxer who once said: “I like the USA.” The two known suspects in the attack on the Boston Marathon — one killed, one on the loose — are brothers with a background in the separatist Russian republic of Chechnya, law enforcement officials told NBC News. The suspect at large early Friday was Dzhokhar Tsarnaev, 19, born in Kyrgyzstan, who became a naturalized American citizen on Sept. 11, 2012, according to documents obtained by NBC News. He had a Massachusetts driver’s license and was living in the...