STARKVILLE, Miss. — She was a cheerleader, an honor student, the daughter of a police officer and a member of the high school homecoming court who wanted to be a doctor. He was a quiet but easygoing psychology student. His father is a well-known Muslim patriarch here, whose personable air and habit of sharing food with friends and strangers made him seem like a walking advertisement for Islam as a religion of tolerance and peace. Today, the young woman, Jaelyn Young, 19, and the young man, her fiancé, Muhammad Dakhlalla, 22, are in federal custody, arrested on suspicion of trying...