Istanbul, Turkey (CNN) ISIS' former minister for health Kefah Basheer Hussein is calm and collected, almost too composed and smug for a captured man. A corner of his mouth twitches in what appears to be an attempt to hide a smirk. He answers "no comment" to yet another CNN question about the murky years when al Qaeda in Iraq went underground only to reemerge as ISIS.

Hussein says he was a high-level ISIS operative -- not a fighter on the battlefield but someone who formed the backbone of the terror group. An intellect in the shadows, he helped ISIS to survive and morph over the years.

CNN exclusively interviewed Hussein after he was apprehended last month by Turkish security forces. The former ISIS minister, who claims that he left the terror group six months ago, is currently in Turkish custody awaiting trial on charges that have yet to be announced.

A rheumatology doctor by training, Hussein, 39, served ISIS and its previous incarnations for over a decade and a half. For the bulk of that time, he remained largely unknown to intelligence agencies, the US military, and the Iraqi government.

Former Iraqi childhood acquaintances, professional contacts, and others connected to Hussein requested to remain anonymous before speaking to CNN about a man whose terrorist affiliations still haunt and threaten them. His story serves as a time capsule of the rise and fall of the radical jihadi agenda that swept through the region.

The US invasion of Iraq

Hussein was raised in Tal Afar, a predominantly Turkmen city in Iraq, situated on one of the main roads between Mosul and the Iraqi-Syrian border. People acquainted with him in high school describe him as "ambitious," "smart" and "very competitive." Like any teenager with big hopes, Hussein set out for the big city when he graduated.

Hussein was training as a doctor in Mosul when the US invasion of Iraq began in 2003. Seeing a foreign occupying army in his country burned him to the core. His family were zealous converts. In the 1970s, their family broke away from the Shia faith of their tribe to Sunni Islam, according to a neighbor from Tal Afar. Adherents of Salafism, a strict and puritanical interpretation of Sunni Islam, their anger was primed as they watched events unfold in Afghanistan and witnessed the fall of Saddam Hussein. The more raids the US military carried out in their hunt for Saddam Hussein's henchmen and, later, in their targeting of the then-nascent insurgency, the angrier Hussein appeared to grow.

"The whole family had a violent reaction," the neighbor says. "When they (Americans) came to Tal Afar, Kefah was part of the first group that was fighting the Americans -- their house was one of the houses that was used for planning operations against the Americans," the neighbor said.

By his own account, Hussein joined Tawhid wal-Jihad in 2004. The group, led by Abu Musab al-Zarqawi, was the ideological precursor to al Qaeda in Iraq. Hussein embraced Zarqawi's ideology.

"We saw many things that happened in front of us, rapes, killings, corruption, the Americans stealing money," Hussein tells CNN.

Hussein says he helped the radical jihadist group by managing health services. It was a role he would play for the better part of a decade across Iraq and Syria as Tawhid wal-Jihad slowly morphed into the so-called Islamic State.

Christian militia fighters from the Nineveh Plain Protection Units (NPU) drive a pick-up truck in Qaraqosh (also known as Hamdaniya), transporting four men, allegedly members of the Islamic State (IS) group that were found inside a tunnel in Mosul.

At the height of the tribal awakening of 2007-2008, when Iraq's Sunni tribes fought against al Qaeda, Hussein had his one and only run in with the authorities.

He was detained briefly in Mosul, according to one ISIS expert. Hussein was accused of being a member of al Qaeda but was released after a short stint in jail and was not mistreated.

Hussein himself was evasive on the subject. "At that time, I had security issues but they didn't know anything about me and I changed my address," he says. Asked if he was in US custody, he responded: "maybe."

A US troops surge prompted by the awakening fighters forced ISIS underground, and Hussein continued his day job at the Ibn Sina Hospital in Mosul. One doctor who worked with him describes him as a "strange" and "distant" figure.

When the ISIS offensive to take over Mosul began in 2014, Hussein emerged from the shadows.

"We weren't sleeper cells, but we weren't participating in the battles for security reasons," Hussein says.

Organ trafficking and blood removal

As ISIS took over the city, a doctor at the hospital recalled Hussein saying: "If any doctors run away, we will execute them."

Hisham al-Hashimi, a leading expert on ISIS in Iraq, told CNN that in Hussein's position, he would have overseen some of ISIS' most horrific activities.

To bolster their blood bank, individuals slated for execution would have their blood removed, in some cases even a kidney extracted. Al-Hashimi said he learned this from his sources as well as people he spoke to who were in ISIS detention.

Kefah Basheer Hussein photographed prior to his arrest.

"It was for their (ISIS') own use, they needed blood. And if they take blood from a prisoner over the course of weeks up until the execution, it would weaken them, make them less likely to struggle. The kidneys, they were for sale," al-Hashimi says.

CNN sources say that they heard about these practices, but had no concrete evidence of it taking place. One former senior official in Iraq's Nineveh province, where ISIS controlled large swathes of land, said that some of the bodies they had recovered from a morgue appeared to have had a kidney removed.

CNN was not aware of the allegation at the time of the interview with Hussein and was not permitted access to Hussein to seek comment on these allegations.

Links to ISIS leadership

Two months after ISIS took control of the city, Hussein's first wife left him, fleeing for the relative safety of Baghdad, according to an acquaintance in Mosul. He pursued his wife relentlessly, sending her threats of abduction, according to the acquaintance who delivered messages between Hussein and his wife.

Hussein took a second wife. It was a marriage of ISIS' top brass. She was the daughter of an influential female doctor, who was prominent within the ISIS cadres. The doctor is believed to be part of the main group that was dispatched to treat ISIS leader Abu Bakr al-Baghdadi after he was injured in 2015.

While Hussein confirmed that he is married and has children, he declined to answer any detailed questions about his family from CNN, saying only, "they exist."

Hussein didn't have a direct relationship with al-Baghdadi, but his second wife did, according to al-Hashimi.

"But these people like him, I call them the black box of ISIS because they have ties to the top tier and to the local leaders. He may not have a direct tie, but he knows important information. And they are trained to be killed and not confess all they know," al-Hashimi says.

"These individuals are fundamental for three main reasons... they are not on any of the terror watch lists, they have charisma and the ability to charm people, and they have complete loyalty to the principles of the caliphate."

A doctor from Deir Ezzor says he met Hussein in the late spring of 2016. He says Hussein was the head of a committee selected by al-Baghdadi, tasked with setting up a hospital in a small village in Syrian between Raqqa -- the ISIS stronghold -- and Deir Ezzor. "I think they picked al-Kasrah [the city between Raqqa and Deir Ezzor] because they wanted to make that they have the capacity to receive high level ISIS members for treatment. I heard that they also improved the equipment there and provided the facility with the best assistance."

An unapologetic view

Throughout our interview, it is clear that Hussein is careful about what he discloses, unwilling to let information slip, but at the same time wanting to appear cooperative. He is oddly apologetic about refraining from answering certain questions. After a string of "no comments" he adds: "I have a trial you know."

Hussein took over the post of ISIS minister of health after his predecessor was killed by a bomb in Mosul, he said. When ISIS began losing control of the city, Hussein moved his base of operations into Syria like many other high-ranking ISIS operatives and began shuttling between Raqqa and Deir Ezzor. "We weren't only serving militants, we were also serving the civilians, people are poor in those areas," he said defending his position.

He is unapologetic in his belief in the Zarqawi doctrine, in ISIS' brutal version of Sharia, or Islamic law. Still he says he was propelled to leave the group after ISIS veered off what he saw as the "righteous path."

"It is a right way of asking people to adhere to Sharia, of course I believe in it... many stuff happened that misdirected the caliphate from its righteous path," he says. "But I always thought about the medical help that I can provide, especially the administrative stuff, so I was a bit late when I left them."

A Syrian who identified Hussein through a photograph knew him as Dr. Omar in the town of al Mayadeen in Deir Ezzor province in June 2017. By Hussein's own account given to Turkish authorities, he left al Mayadeen when fighting intensified. He illegally entered Turkey through the YPG-held town of Tal Abyad.

When pressed by CNN, Hussein is vague about how he left Syria. "Don't ask me how. Don't ask me the route."

After a month in Turkey, he said he secured a fake Syrian refugee ID, but was stopped at a checkpoint while en route to Istanbul.

Alongside him was another Iraqi man who Turkish authorities identified as being one of the men under his command. On that same day, Turkish authorities detained half a dozen foreign ISIS members, including a Dutch and a British national who were on a red notice alert, essentially an international arrest warrant.

Security forces routinely round up hundreds of suspected ISIS members -- ranging from the lowly fighters to logistics organizers to men like Hussein who make up the terror group's brain trust.

Hussein says he did not expect ISIS to lose its so-called "caliphate."

We have defeated ISIS militarily, but only in the sense that there are no ISIS flags, no buildings that they can openly claim as their own... But ISIS is everywhere in Iraq and Syria. Hisham al-Hashimi

"The caliphate is not just in Iraq and Syria. It is in Khorasan (a historic region in central Asia), Libya and the Philippines currently and also in Africa."

Asked if he wants ISIS to survive, Hussein again answers "no comment."

The US military has long said that an organization can only be eliminated when its middle leadership is destroyed. But when it comes to ISIS, al-Hashimi estimates that only about 30% of the dozens that make up this vital cornerstone are known. And that is precisely what makes it so dangerous.

"We have defeated ISIS militarily, but only in the sense that there are no ISIS flags, no buildings that they can openly claim as their own," al-Hashimi adds. "But ISIS is everywhere in Iraq and Syria."

Hussein likely won't have a role in the next ISIS incarnation.

But when it comes to this deadly, radical ideology, it's not a matter of if but when its other ghosts emerge from the shadows.