By Daniel Wallis (Reuters) - The Parents of an U.S. aid worker held hostage by Islamic State militants on Sunday released photographs of their son and parts of a letter he wrote them from captivity in which he says he is scared to die but at peace with his belief. Peter Kassig, 26, was taken captive a year ago while doing humanitarian work in Syria, his family has said. He was threatened in an Islamic State video issued on Friday that showed the beheading of a British aid worker. Ed and Paula Kassig of Indianapolis, Indiana, appealed for his release on Saturday in a video message. ...