The mother of Abdul-Rahman Kassig, formerly known as Peter Kassig, reached out to his captors on social media Wednesday night.
Sources close to the family confirm a letter posted on a @PaulaKassig Twitter account is authentic.
Abdul-Rahman was captured by the Islamic State in October 2013. He founded an aid organization called SERA, which provides humanitarian aid to Syrians.
ISIS threatened to kill Abdul-Rahman in a video released last Friday.
Hundreds gathered to pray for him during a vigil at Butler University last night. They asked ISIS to show Abdul-Rahman mercy and release him.