(CNN) -- Saying she's an "old woman" trying to find out the fate of her only son, the mother of American hostage Abdul-Rahman Kassig has sent a tweet meant for ISIS leader Abu Bakr al Baghdadi.
"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.