Among the many unanswered questions about the two Tsarnaev brothers accused of the Boston Marathon bombing is why, days after the attack, they were heading to the suburb of Watertown and its manicured lawns and tulips when police picked up their trail and began a chase. Investigators want to know what drew the accused bombers to the cluster of side streets in the blue-collar suburb, far from any major thoroughfare, especially if the brothers were on the run after their images had been shown on television by the FBI and after they had allegedly murdered MIT Police Officer Sean Collier....