Complex sentence: At the time of the bombings, Dzhokhar was a student at University of Massachusetts Dartmouth who became a naturalized U.S. citizen on September 11,2012, seven months before the bombings.
Split into simpler sentences: At the time of the bombings, Dzhokhar was a student at University of Massachusetts Dartmouth. He had become a naturalized U.S. citizen on September 11,2012, seven months before the bombings.