Inside a low-slung concrete building ringed by blast walls and razor wire, unrepentant ISIS fighters awaiting execution told Fox News they killed and maimed innocent civilians under "license from God" while fighting alongside radical Americans and vowed the terror army can never be truly defeated.

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Fox News was recently given exclusive access to prisoners, who spend their days behind yellow, steel doors and freely admit to killing hundreds of people as leaders of ISIS cells around the embattled country. Iraqi officials allowed the interviews in between interrogations of their own, aimed at understanding how the terror group operates.

"I walked away, and called a number on my phone." — Abu Omar, describing how he detonated a car bomb that killed 100

Abu Omar, 25, told Fox News he oversaw a cell in Baghdad and, by his own admission, carried out countless bombings.

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After joining the terror group, he was initially assigned to guard a checkpoint in Mosul. There, he arrested women who weren’t properly veiled. For their offenses, they faced public lashing.

Omar told Fox News he was plucked from the post and trained in deployment of explosives. With his new skills, he moved up the ranks quickly before being sent to Baghdad to lead a sleeper cell charged with launching attacks against the public, he said.

He calmly described how he would pick up a car laden with explosives, always from someone he hadn’t met before, and park it at a busy, pre-scouted location.

"I walked away, and called a number on my phone," he said.

As Omar melted into teeming crowds and detonated the bomb, all that remained was to wait to learn how many innocent men, women and children his twisted handiwork killed or maimed.

One bomb killed more than 100 people, including 30 women and children. Omar, who estimates he had a role in the deaths of 300, calmly described the screams and the chaotic aftermath.

Now facing a death sentence, Omar -- despite a rote admission of remorse utterly lacking in conviction -- appears untroubled by his deadly deeds. He claimed he had a license from God to kill in the name of Islam, and still believes ISIS is justified in torturing and burning prisoners of its own.

It’s so difficult to get informants inside ISIS that this facility has proven a gold mine for information. Intelligence gathered here is used to help the anti-ISIS coalition target airstrikes and predict the behavior of the black-clad jihadist army. Most of its occupants face the gallows when their usefulness ends.

The interrogation program was designed by U.S. trainers. Guards told Fox News it does not include torture, and that the worst treatment they can inflict on prisoners is sleep deprivation.

Karim, 27, was ISIS' key man in Baghdad for distribution of another type of bomb, the Improvised Explosive Device, or IED. Like Omar, he showed no remorse as he recounted fighting and even killing Americans. He described his battle as a global war, and said that during his time in ISIS, he believed the caliphate would take over the world.

Alarmingly, Karim spoke of Americans he had met within the terror group. Two or three, he said, had been kept separate from the others and were part of a larger group of foreigners in charge of threats against the West.

"They were more radical than the others," he said of the Americans.

Karim described the sleeper cell in which he served as a hierarchy where communications up the chain of command were kept purposefully controlled. The system is much like the one used by Al Qaeda, where meetings are arranged at a set time and place and if someone doesn’t appear -- they consider it "burnt" and the operation is scuttled.

The cold and murderous resolve of these captured ISIS fighters makes clear to officials -- both U.S. and Iraqi -- that even once the terror army is defeated, its members will morph into a stubborn and deadly insurgency similar to the one that dogged Iraq after the fall of Saddam Hussein. Should they be driven from strongholds such as Mosul and Raqqa, Syria, ISIS diehards will continue, Karim vowed, to attack from the shadows.