An American aid worker threatened to be beheaded by the Islamic State group told his parents in June that he is "scared to die" and disheartened by the heartache that his imprisonment is causing his family.

Abdul-Rahman Kassig, 26, was working to help refugees in Syria when he was captured by the notorious terrorist group known as ISIS in October 2013. He managed to send his family a letter that they received on June 2 talking about his potential execution, his parents said on Sunday.

"I am obviously pretty scared to die but the hardest part is not knowing, wondering, hoping, and wondering if I should even hope at all," Kassig said in the letter, according to his parents, reports the Associated Press. "I am very sad that all this has happened and for what all of you back home are going through. If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."

The letter continues: "In terms of my faith, I pray every day and I am not angry about my situation in that sense. I am in a dogmatically complicated situation here, but I am at peace with my belief."

Following the beheading of British aid worker Alan Henning last week, ISIS released a video warning that Kassig would be their next victim. In response, his parents pleaded for their son's life to be spared in a video released Saturday.

"Most of all, know that we love you, and our hearts ache for you to be granted your freedom so we can hug you again and then set you free to continue the life you have chosen, the life of service to those in greatest need," Paula Kassig said in the video, according to ABC News. "We implore those who are holding you to show mercy and use their power to let you go. 

Although Kassig was named Peter, his family requested that he be identified as Abdul-Rahman because he has voluntarily converted to Islam sometime during his capture.

His family also said that his is a former Army Ranger, founded an aid organization called Special Emergency Response and Assistance (SERA) in Turkey.