Police in the U.S. are still trying to uncover the motive behind the murder of the most decorated sniper in the history of the U.S. military. 

Chris Kyle, who wrote "American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History," and his friend Chad Littlefield were shot dead at a firing range in Glen Rose, Texas on Saturday, allegedly by a man who Kyle was trying to help overcome psychological trauma stemming from the field of battle.