In the video, which Mueller's family released to ABC News, Mueller is seen wearing a black hair covering, looking thin compared to photos of her from before she was captured.
"My name is Kayla Mueller," she said in the video, intended to be proof of life for her family and friends back in the US.
"I need your help," she continued. "I've been here too long, and I've been very sick. It's, it's very terrifying here."
See images of Kayla Mueller:
Kayla Jean Mueller -- ISIS Hostage, Kayla Mueller
Kayla Jean Mueller -- ISIS Hostage, Kayla Mueller
Courtesy: Mueller Family
#Obama confirms death of American #ISIS #hostage Kayla Mueller; Private Message from ISIS sent to Parents of ISIS Hostage, Kayla Mueller, Confirms Her Death http://read.bi/16QUgjt
Statement from the White House on the death of Kayla Mueller
Parents of #ISIS hostage Kayla Mueller urge her captors to contact them. http://t.co/oYOzFBF62r http://t.co/DTkx9od38u
The Mueller family have released new pictures of Kayla. http://t.co/vk7warF4OQ
The parents of American Islamic State (ISIS) hostage Kayla Mueller, who the extremist group claims was killed Friday by a Jordanian airstrike on the Syrian city of Raqqa, are hopeful that their daughter is still alive. Carl and Marsha Mueller said in a statement on Friday, "This news leaves us concerned, yet, we are still hopeful that Kayla is alive." Mueller's parents urged ISIS to contact them privately, and to treat their daughter as a guest.
.@ejmontini: For Kayla Mueller's family, waiting is not a game http://t.co/TvtOCrgFJv http://t.co/GEaSQNdKKv
Prescott stricken by loss, uncertainty of Kayla Mueller's fate. via @RebekahLSanders
Family, others kept Kayla Mueller's capture a secret to protect her, @KarenBrown3TV reports http://t.co/bHlvjWYAqw http://t.co/noUUfRUS6m
Family of hostage Kayla Mueller breaks silence after #IslamicState claims: http://t.co/Dv5w7uOFwW | http://t.co/iSNja0KShX
JUST IN: ISIS sent Muellers an e-mail saying their daughter was dead and included a photo, David Martin reports; cause of death inconclusive
A local Palestinian man holds up a poster of slain ISIS hostage Kayla Mueller who worked in Palestine and was an activist with the International Solidarity Movement- a movement dedicated to supporting a free Palestinian state. In the Palestinian West Bank village of Bilin, west of Ramallah, the 10th anniversary of the popular resistance movement against the Israeli occupation was held on Friday afternoon. Israeli soldiers invaded the village on Friday, stopping protesters from marching into the olive orchards. Israeli soldiers assaulted Palestinians and international activists, beating them, spraying them with a chemical orange agent, as well as using hundreds of tear gas canisters. Live ammunition rounds were also fired at protesters, including sound bombs thrown at the feet of journalists. Journalists who were filming the protests were assaulted by Israeli soldiers as well. The village gained global notoriety when a decade ago locals as well as international activists would gather and organize weekly protests on Fridays, marching towards olive orchards and lands that were confiscated by the Israeli government to build illegal settlements and the separation barrier in the West Bank. The village gained even more popularity after one of its locals, Emad Burnat, filmed the Oscar-nominated film, Five Broken Cameras in which he showed the struggle of the Palestinian village against Israeli aggression. Scores of people, both Palestinian and international have been injured during weekly protests, the most infamous of whom was Basem Au Rahma who was killed in 2009 when a tear gas canister struck him in the chest. Two years later, in 2011, Jawaher Abu Rahma also died during a weekly protest after being hit with a tear gas canister. (Photo by Anna Ferensowicz/Pacific Press/LightRocket via Getty Images)
Palestinian protesters hold placard to protest against 'terrorism' on February 13, 2015 before a demonstration against Jewish settlements in the West Bank village of Bilin, west of Ramallah. The poster shows a picture of US aid worker Kayla Mueller, who died as a hostage of Islamic State (IS) group jihadists. (Photo credit ABBAS MOMANI/AFP/Getty Images)
Palestinian protesters hold placard to protest against 'terrorism' on February 13, 2015 before a demonstration against Jewish settlements in the West Bank village of Bilin, west of Ramallah. The posters show pictures of US aid worker Kayla Mueller, who died as a hostage of Islamic State (IS) group jihadists. (Photo credit ABBAS MOMANI/AFP/Getty Images)
The video echoes later ISIS productions that are known for their slick editing and high quality. Chris Voss, a retired FBI chief hostage negotiator who viewed the video, explained why this is.
"You look at this video, and right away you can see a number of things. Basically from a pure physical health standpoint, she's not in bad shape physically. They're letting us see that," Voss told ABC. "They want us to see that overall, she's not in bad shape. They probably put makeup on her before they shot the video. They produce these the same way any media company produces videos."
Voss speculated that ISIS had Mueller rehearse what she was going to say.
"They probably rehearsed that a number of times," Voss said. "I would imagine they shot that anywhere from no less than five times, maybe as many as 15 times. They rehearsed her. They got the lighting right. They controlled what's in the background. They controlled everything they said. Everything she said. ... They want to put enough out there to start a negotiation. And that's what this is intended to do."
One of the terrorists who took Mueller hostage in Syria emailed the video to one of her friends in the US, who then forwarded it to the FBI. The video came about three weeks after Mueller was taken.
ISIS was hoping to earn ransom money from holding Mueller hostage. But the US has a policy against paying ransoms to terrorist groups, who often kidnap Westerners to extort money they use to fund their operations.
This video wasn't the only message Mueller sent to the outside world while she was in ISIS captivity.
"Just the thought of you all sends me into a fit of tears," she wrote. "If you can say that I have 'suffered' at all throughout this whole experience it is only in knowing how much suffering I have put you all through; I will never ask you to forgive me as I do not deserve forgiveness."
Mueller's family also received an audio clip in 2014. She explained what her captors wanted in exchange for her safe return.
"Mom and Dad, I still am remaining healthy," she said in the recording, according to ABC. "You should have already received the three answers to the proof life questions you provided. Those detaining me are demanding an exchange of Dr. Aafia Siddiqui's release for my release. If this is not achievable, they are demanding 5 million euros to ensure my release."
ABC News spent two years investigating the kidnapping and imprisonment of Mueller at the hands of ISIS. Their special report is airing on "20/20" Friday at 10 p.m.