A skeletal portrait of the Sterling, Va., man federal authorities have accused of obstructing a terrorism investigation began to come into focus on Monday — that of a directionless youth who converted to Islam at the end of high school, married a significantly older Muslim woman, had a child who died as an infant, and became increasingly religious until an arrest Friday that allegedly included him destroying a computer thumb drive that authorities suspect may have contained evidence of terrorism-related activity. In the months before that arrest, court filings allege, Sean Andrew Duncan, 21, who will appear in federal court...