Kassig's mother plea to ISIS leader: 'I am an old woman, and (he) is my only child'

"I am trying to get in touch with the Islamic State about my son's fate," she says in the tweet, which is a photograph of a letter written to al Baghdadi.

"I am an old woman, and Abdul Rahman is my only child.

"My husband and I are on our own, with no help from the government. We would like to talk to you. How can we reach you?"

Abdul-Rahman Kassig, whose given name is Peter, was detained on October 1 in eastern Syria while doing humanitarian work. ISIS has threatened to kill him next.

Since the announcement -- made in an ISIS video showing the apparent beheading of of British aid worker Alan Henning -- Kassig's has issued repeated pleas asking his captors to show mercy and free him.

Over the weekend, they released a YouTube video, imploring his captors to "use their power to let our son go."

Wednesday night, Kassig's parents attended a vigil at Indiana's Butler University, Kassig's alma mater.

'I am an idealist'

Kassig, 26, had already finished a tour in the Middle East as a U.S. Army Ranger stationed in Iraq. But he felt compelled to go back to the region to help Syrians caught in a bloody conflict that has killed roughly 200,000 people

He expressed his fear in captivity in a letter sent to his parents in June.

"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 wrote.