Proof of Life: ISIS Hostage Kayla Mueller's Heartbreaking Never-Before-Seen Video Message From Captivity

— -- The .mp4 video clip was only 10 seconds. Long enough for her family to recognize the young American woman in a black head scarf and green hijab but not long enough to identify where she was or who was holding her hostage in Syria in 2013.

"My name is Kayla Mueller," she began in a clear voice, her eyebrows arched upward, in apparent stress, above her glassy eyes, "I need your help."

Mueller, a 25-year-old humanitarian aid worker, was a hostage of a terrorist group the world would come to know as the worst in history — but she didn't say any of that.

At 10 seconds and 22 megabytes, her proof-of-life video was small enough for the hostage takers to send by email from Syria. She had been missing from the war-torn city of Aleppo for almost a month by the time this ISIS-made video eventually made its way to her parents in Prescott, Arizona, on Aug. 30, 2013.

The video, provided by the Mueller family to ABC News for Friday's "20/20" broadcast of "The Girl Left Behind," is the only known image of Kayla Mueller in ISIS captivity and had never been shown publicly before now.

"You just go into almost a catatonic state, I think. You can't even stand up," Carl Mueller told ABC News in a recent interview, describing his reaction three years ago to first seeing his daughter in the video.

FULL BRIAN ROSS REPORT: "The Girl Left Behind."

Few at the time had ever heard of ISIS — a group originally known to the U.S. as "al-Qaeda-Iraq" — or knew that it was violently breaking off its alliance in Syria with a franchise of core al-Qaeda in Pakistan, which Osama bin Laden founded. ISIS needed cash, and Kayla Mueller became one of the first Westerners it kidnapped in Syria's war-torn streets for millions in ransom.

"I've been here too long, and I've been very sick. It's, it's very terrifying here," she said to the camera before the image, which showed only her covered head and shoulders, abruptly stopped.

Mueller had clearly lost weight since she was abducted by a group of gunmen on Aug. 4, 2013, from a Doctors Without Borders vehicle not far from a hospital in Aleppo, Syria, run by the group's branch in Spain.

"I saw how thin she looked, but I saw that her eyes were very clear and steady," said her mother, Marsha Mueller. "It broke my heart, but I also saw her strength."

The proof-of-life video was intended to serve a few basic purposes, according to Chris Voss, a retired FBI chief hostage negotiator who examined the video.

"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. They want us to see that overall, she's not in bad shape," he told ABC News. "They probably put makeup on her before they shot the video. They produce these the same way any media company produces videos."

The reason for showing her in good health was that, to ISIS, Kayla Mueller was a commodity. "This is an opening offer. This is, 'We want to talk,'" Voss explained.

"They probably rehearsed that a number of times. 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 without raising the threat level. They want to put enough out there to start a negotiation. And that's what this is intended to do," he said.

The Muellers instead put their faith in the nongovernmental aid organizations for which their daughter worked — the Danish Refugee Council, Support to Life and the NGO Forum, a collective of many aid groups — which told them the U.S. government had stepped in to take care of things and would get Kayla Mueller home.

Her family trusted all "like sheep," Carl Mueller now says.

Support to Life was helpful to the Muellers, but it is a small organization with limited resources or knowledge of how to handle a hostage case, they say.

Their faith in aid groups and the government meant that the Muellers did not begin negotiations with the hostage takers for 10 months, when Doctors Without Borders turned over an ISIS email address, two months after the aid group received it from some of its workers freed from captivity.

Negotiations began with the hostage takers on May 23, 2014. Most American families of ISIS hostages received few emails from the hostage takers, but the Muellers received nine.

Soon after that, on May 29, proof of life came with Kayla Mueller's voice — but not her face -- in an audio clip.

"Mom and Dad, I still am remaining healthy. 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," she said.

And then, abruptly, what would be Mueller's last spoken word to her mom and dad: "Goodbye."

ABC News' Rhonda Schwartz, Lee Ferran, Matt McGarry, Engin Bass, Alex Hosenball, Cho Park and Esther Castillejo contributed to this report.