Baghdad, Iraq(CNN) Sameem Sulaiman shuffles in, the cuffs usually around his ankles and wrists removed for our interview. He agreed to speak to CNN, but not once during the interview did he make eye contact, or even look up, unwilling to speak directly to a woman.
Sulaiman had never met ISIS leader Abu Bakr al-Baghdadi, though his path to ISIS also stemmed from being held in U.S detention in Camp Bucca, a U.S.-run detention center in Iraq.
In mid-2008, U.S. forces picked up Sulaiman in Mosul, Iraq's second-largest city. He was in charge of salary payments for the Sunni group Jaish al-Mujahedeen, one of the many insurgent groups that carried out deadly attacks against American troops.
Sulaiman was released a year and a half after his detention.
"They didn't have any evidence against me," he recalls. "I went back to Mosul, and the situation had changed. The Americans had withdrawn. So I started working with my brothers in construction."
By 2013, like most of Mosul and Iraq's Sunni population, Sulaiman felt disenchanted and alienated, incensed by the Shia-dominated government's oppression of Sunnis and mass roundups by Iraqi security forces in the city.
One day, out of the blue, a man he had met at Camp Bucca contacted him.
The man was looking for someone to help Abu Abdul Rahman al-Bilawi, a former special forces officer in former Iraqi leader Saddam Hussein's army who later established ties to al Qaeda in Iraq.
He was told that "Bilawi said, 'I need you to help me out,' " Sulaiman says. " 'I got out of prison a while ago, and I am with the Islamic State.' "
Sulaiman says that at the time, he didn't know that al-Bilawi was one of al-Baghdadi's top military commanders, a member of the ISIS war cabinet. Al-Bilawi is widely believed to be among the hundreds of people who escaped from the Abu Ghraib detention facility after it was attacked by ISIS terrorists in July 2013.
"He said I just need you to rent a house for me, I need to get married, and you to look after my family and to stay at the house. In case there is a search, the house needs to be in your name." Sulaiman says of al-Bilawi's requests. "I said no problem. And I didn't ask about specifics of what he did."
Sulaiman rented a two-story house and moved his family into the bottom floor.
"I would drive him around Mosul." Sulaiman says. "He would always wear a suicide belt and carry a pistol. But even at checkpoints, he wasn't afraid, he didn't care. He had a look of authority. At the checkpoints, troops assumed he was an officer with security forces."
Despite the relationship between the two, Sulaiman says al-Bilawi was highly secretive.
"He wouldn't let me see anyone who he was meeting." Sulaiman remembers. "We'd coordinate our meetings via text message. We'd change the SIM cards and our mobile phones weekly."
Eventually, Sulaiman knew that something significant and imminent was in the works.
"I didn't have specific details, but he told me if there is an operation, don't leave."
Four days before ISIS stormed into and took over Mosul, federal police raided Sulaiman and al-Bilawi's home, killing al-Bilawi. Police said he was reaching for his suicide belt just as they gunned him down.
Senior federal police officers tell CNN that had Iraqi security forces commanders stationed in Mosul at the time taken the intelligence gleaned seriously, Mosul may not have fallen to ISIS, at least not so quickly.
But it was not just an intelligence failure that allowed ISIS to thrive and grow.
Iraq's Sunni population had grown increasingly disenchanted with the Shia-dominated government, accusing it of indiscriminate mass roundups of Sunnis and alienating the population. For years, Sunnis had been demonstrating against the government as ISIS' predecessor ISI -- the Islamic State of Iraq -- re-emerged in strength after U.S. troops were withdrawn.
The ISIS offensive in Mosul is one of the Iraqi military's biggest embarrassments, as Iraqi troops and police dropped their weapons and ran from their posts as militants arrived in June of last year.
But according to a U.S. official, up to 25,000 Iraqi troops are expected to head back in April or May to take part in an assault.
Iraqi Prime Minister Haider al-Abadi also told the BBC in February that he was sure Iraqis would retake the crucial northern city.
"We are now planning an offensive on Mosul in the coming few months," the Prime Minister said. "We have to prepare for it carefully, because the only choice we have in Mosul (is to win). We have to win in Mosul to keep (ISIS) out," he said.
Also in federal police custody is the main Baghdad-based ISIS supplier for explosive material and suicide belts.
Ammar Ali Khalil, in a yellow prison suit, speaks quietly. He too is forthcoming about the violence he helped perpetrate.
"In my mind, to have an Islamic state, it was the right path," says Khalil, who was also in charge of special operations for the Baghdad emirate.
He was also involved in the insurgency against U.S. forces, but he was with al Qaeda, moving bomb-making materials in the al-Doura neighborhood in southeastern Baghdad. As al Qaeda evolved to become ISIS, he swore fealty as well.
"I was the one who prepared the suicide vests and placed it on the person. I was also strategizing targets and operations."
He says he personally organized and dispatched 19 suicide bombers, among them a German and an Australian.
"A top military commander in the Baghdad emirate would contact me that a suicide bomber is ready. The operation would be preplanned. I would receive them in locations close to Baghdad and either take them to our safe house or straight to the target."
Those bombers killed dozens of people, and most of the targets were in Shia neighborhoods.
READ: Author's journey inside ISIS: They're 'more dangerous than people realize'
Khalil, who will most likely receive the death penalty, says he regrets his actions.
But he admits that if he had not been caught, he would have continued to carry out attacks. He was establishing a makeshift bomb-making factory in the heart of Iraq's capital, where federal police say they found more than a dozen suicide vests and around 250 bombs.
He pauses when we ask what he would say to his three children if he could see them now, and then finally says, "I cannot speak about my children. ... I am sorry."