The prisoners were marched from their makeshift jail down the street to a house that Mosul's police have turned into a temporary detachment.

They were led into what used to be a family's living room, with the green curtains drawn shut.

Officers brought the men, separately, to speak to CBC News, in what is a rare opportunity to gain insight into why Iraqis joined the world's most notorious militant group. The prisoners were blindfolded to prevent them from identifying their guards.

"My name is Juma Ibrahim," said one of the men, his head bowed and eyes covered by a thin piece of white cloth. His hair had recently been shaved off. "I am a member of ISIS."

Men suspected of ties to ISIS are held in a former home in eastern Mosul. Militants destroyed most police stations so security forces now operate out of houses. (Derek Stoffel/CBC)

On a recent trip to Mosul, where the battle against the self-proclaimed Islamic State continues, CBC News was given access to two young Iraqi men who police say were ISIS members. Both were awaiting transfer to a larger detention facility for militants south of the city. Neither had been charged.

The prisoners were being held in east Mosul, which was liberated by Iraqi security forces in January. Across the Tigris River, the military operation to defeat ISIS in the western side of the city continues.

The makeshift jail consisted of a few small rooms, guarded by officers, in another house on the same block. A brief look inside one room saw at least seven men, most kneeling on the cramped floor. The room reeked of feces and body odour.

Qutaiba Salem, 25, said he joined ISIS in Mosul to pay for medicine for his mother. (Derek Stoffel/CBC)

Both Ibrahim and the first detainee we met, Qutaiba Salem, admitted they joined ISIS, but both denied carrying out acts of violence against residents of Mosul.

"I only joined them for money to buy medicine for my mother," said Salem, 25, who said he was an electrician living in the western Mosul neighbourhood of Wadi al-Hajar before ISIS stormed the city in June 2014.

Ibrahim, who said he'd been working in construction prior to the arrival of the jihadist group, made a similar claim.

"My sister, she has cancer. And I didn't have money," he said. "I didn't have treatment. I visited [ISIS] and I asked them for treatment. They said, 'We don't give you treatment unless you join us."

'I'll kill you if you don't tell the truth'

The atmosphere in the interview room was tense, especially when Maj. Mezher Sadoon, who heads the emergency response division of Mosul's police force, was present.

Sadoon interrupted the first prisoner, Salem, and yelled at him: "Hey, boy, tell the truth! What did you see?

"Don't lie."

Maj. Mezher Sadoon, who commands the emergency response division of Mosul’s police force, gestures to where he was shot by a suspected al-Qaeda operative in 2004. (Derek Stoffel/CBC)

During our interview with Salem, Sadoon jabbed his fingers into Ibrahim's back repeatedly. The officer slapped him in the back of his head a few times. At one point, Sadoon motioned to us to turn off our camera.

"Son of dog," Sadoon snarled at Salem when the interview resumed. "Tell the truth. Even if they fire me, I'll kill you if you don't tell the truth."

But the prisoner's story didn't change.

"Sir, believe me, I didn't see anything," he responded. "I swear I was only cooking."

Machine gun ammunition was stored in this house in western Mosul where Iraqi security forces say ISIS fighters were holed up for several weeks. (Derek Stoffel/CBC)

Salem maintains he spent two years working in the kitchen of a Mosul house that five ISIS fighters called home. He said he was paid about $125 a month (a claim that raised the ire of officers in the room who say that salary wouldn't cover the cost of the medicine his mother needed).

Sadoon is unapologetic for his treatment of the confessed ISIS members. He's spent his career fighting militants in Iraq, including members of al-Qaeda and now ISIS.

'They lie, all of them'

The major, with his hair cropped flat and painter's-brush moustache, was shot in the face by a suspected al-Qaeda operative in Mosul in 2004. Metal pins hold his jaw in place.

"All of [the prisoners] say, 'We were simply cooks,' or make some other excuse," Sadoon said. "They lie, all of them. It's just like ISIS taught them all the same thing. They all participated in battles and they are connected to each other like a chain."

An Iraqi special forces commander holds a captured ISIS flag upside down to obscure the militant group's message. The army says it has relied on a network of neighbourhood informants to help weed out ISIS supporters. (Derek Stoffel/CBC)

Iraqi security forces have detained more than 1,200 people suspected of having ties to the militants, including boys as young as 13, according to Human Rights Watch.

In a recent report, the advocacy group condemned "deplorable prison conditions" that may have led to the deaths of four detainees, due to a lack of "proper medical conditions and poor care."

Many of those accused of supporting ISIS were turned over to police by friends or relatives. Security officials said informants in neighbourhoods throughout Mosul are a valuable tool in finding suspected militants.

Mosul residents wait at an assembly point for those who've fled the city. Security officials screen men and boys to try to determine possible links to ISIS. (Derek Stoffel/CBC)

Tens of thousands of Mosul residents have fled their homes during the military campaign over the past six months, and the authorities have developed a system aimed at weeding out ISIS supporters among them.

Men and boys are separated from their families while security officials check their names from a master list of suspected ISIS members or backers. A senior Mosul police officer told CBC News the information used to prepare that list often came from neighbourhood informants.

Juma Ibrahim, the second ISIS member we spoke to, said he turned himself in to police in early February.

Given a gun to enforce ISIS rules

The tension in the room subsided during the interview with Ibrahim. Maj. Sadoon wasn't there for most of the conversation. He never once laid a hand on the prisoner.

Ibrahim, 23, admitted he joined ISIS, swore an oath to the caliphate, and then received extensive training. He was issued an AK-47 assault rifle when he started his job as a police officer at a small hospital in the town of Qayyarah, south of Mosul.

He said a typical day involved making sure there were no disputes between patients and doctors at the clinic. He was also responsible for enforcing the ISIS dress code: women were to keep their faces covered at all times and men had to wear short trousers to follow Islamic tradition.

He said he never fired his AK-47, except during training.

Ibrahim recalled one bloody day when his superiors discovered four men who attempted to swim the Tigris River in Mosul.

"They tried to cross the river and go to liberated areas," he said dispassionately. "They caught them and killed them in public in front of my eyes."

An Iraqi police officer leads Juma Ibrahim, who admitted joining ISIS, to meet with journalists at a makeshift detachment in eastern Mosul. (Derek Stoffel/CBC)

When asked what punishment he should face, the construction worker-turned-ISIS member said he should go to prison "for a month or two months … but not forever."

And before he was led back to the makeshift jail cell, Ibrahim said he had a message he wanted to send halfway around the world.

"Dear Canada and Canadians: ISIS is a ghost. They care only about killing and they are non-believers," he said, the volume of his voice rising.

"Never let anyone from ISIS enter your country. Never negotiate with them. If ISIS came to Canada, Canada would be like Mosul now: destroyed. If you know anyone supporting ISIS in Canada, kill him."