Although U.S. hostage Kayla Mueller was tortured, verbally abused, forced into slave labor for ISIL commanders in Syria and reported raped by the group's top leader, Mueller's fellow hostages said in ABC News' "20/20" broadcast, "The Girl Left Behind," that Mueller never surrendered hope or her Christian faith. The Prescott, Az., humanitarian aid worker was a courageous 26-year-old who inspired the other hostages, they said. She was killed during February 2015, during one of a dozen Jordanian airstrikes in Al-Raqqah.

Four former hostages, who shared cells with Mueller, spoke publicly for the first time about their ordeal in the TV news segment. They relayed that Mueller selflessly put the welfare of fellow captives above her own, and even stood up to executioner Jihadi John to defend her Christian faith.

On Saturday, a new state-of-the-art playground was erected in Mueller's hometown in her honor, reports ABC.

Sen. John McCain, R-Arizona, who survived Communist torture in the "Hanoi Hilton" as a Vietnam War POW, told well-wishers at the rainbow-colored playground he doesn't call many people "hero," but Mueller was one.

"I didn't know Kayla, but I sense in every account of her ordeal, the true meaning of humility," McCain said in a short speech.

Mueller was kidnapped by armed gunmen on Aug. 3, 2013, while traveling in a Doctors Without Borders vehicle in Aleppo, Syria. She died 18 months later still a prisoner of ISIL terrorists. U.S. officials denied an airstrike killed her, but her parents still do not know how she died or how the White House was able to confirm her death during February 2015.

She adored the Syrian refugee children she worked with in southern Turkey, her friends have said.

The only period in Mueller's 18 horrifying months as an ISIS hostage when she wasn't subjected to some form of torture, verbal abuse, prolonged isolation, sensory deprivation, stress positions, forced labor or sexual assault before she died in captivity was the six weeks she was held at an abandoned oil refinery in Syria, with other Western hostages in 2014, reports ABC.

Three Westerners released by ISIS and a Yazidi teenager, who escaped captivity, provided eyewitness accounts to ABC News of Mueller's strength, selflessness and will to survive amid her considerable suffering, including details she gave them of her treatment when she was completely alone for most of her confinement by the terrorist group.

"They would scream at her, and blame her for everything that America has done in the world," Frida Saide, one of three women from Doctors Without Borders who shared a cell with Mueller at the oil refinery, told ABC News in the interview.

"They picked her apart," said Patricia Chavez, one of the other Doctors Without Borders aid workers held with Mueller.

Fellow hostages also shared that Mueller often prayed for them to escape and to survive.