(CNN)Nadia was just 15 when ISIS fighters swarmed into her homeland. She says she was kidnapped, bought and sold by men who told her they owned her, and repeatedly raped by them.
Fighting to bring ISIS to justice for war crimes against Yazidis
Three years on, the traumatized Yazidi is finally free of her captors, but remains trapped in a living nightmare, fearing the ISIS militants could still get to her, or worse, kill members of her family.
The frail 18-year-old former slave doesn't even know if her mother, father and brother are still alive.
At a meeting in the Kurdistan region of Iraq, she hides behind her cousin, one hand clenching her relative's shirt, the other holding tight to her own dress. Shell-shocked, she cannot remember how many younger siblings she has, and struggles to recall their names.
Nadia (not her real name) was among thousands of Yazidis captured by ISIS in August 2014 when the terror group launched an assault on Sinjar, then home to more than half a million members of this minority group.
In the days that followed, ISIS fighters split up families, executed the men and declared the women their slaves.
Nadia and and her older sister Amira were separated from the rest of the family. Their mother was pregnant at the time, and Nadia doesn't know where she - or her baby - is now.
The sisters were bused to several Iraqi towns like cattle, she recalls. They were taken to Mosul and locked up in one of Saddam Hussein's former palaces.
It was here, Nadia says, that she witnessed an incident that still haunts her: ISIS militants took a child away from his mother as she screamed for him. Her captors responded by hitting her on the head. Nadia still doesn't know what happened to the pair at the hands of the terrorists.
"They would tell us we sold you, we bought you," she says. "There were days we would wish we would die so no one says they sold us ... Beating and everything else is bearable, but not selling."
An Iraqi Kurdish investigator who asked to be identified as Shaima has heard stories like Nadia's dozens of times as she interviews freed Yazidis. "Some of them are eager to tell their story. Some of them are reluctant because they can't go into the details -- it's difficult for them," she says.
Shaima is a member of the Daesh Criminal Investigations Unit (DCIU), a team of Iraqi Kurdish and western investigators who have been operating secretly in Northern Iraq, for more than two years, collecting evidence of ISIS' war crimes.
The DCIU works for the Commission for International Justice and Accountability (CIJA), an organization based in Europe, its exact location kept secret for security reasons.
Their task in Iraq is dangerous -- the terror group remains at large despite losing most of the territory it once controlled.
"We will put the danger aside," Shaima told CNN. "The purpose we are serving is much bigger than to think about the danger."
Investigators have collected thousands of documents, phone records and videos left behind by the militants after they ravaged much of northern Iraq.
Earlier this month, CNN conducted exclusive interviews with the team in Iraq and toured its European office.
Bill Wiley, a Canadian criminal lawyer, is CIJA's founder. He has been investigating war crimes for more than two decades, in places like Rwanda, the Balkans and in the Eastern Congo. He says he's never seen anything like the "creativity" of ISIS' crimes, including murder, torture and sexual offenses.
"In creativity they are absolutely unique, the throwing of homosexuals or suspected homosexuals off tall buildings, sexual slavery in an organized way," Wiley told CNN.
And so callous are the ISIS captors, he adds, that they have been known to execute Yazidi women judged too old to exploit as sex slaves.
"The utter depravity of Islamic State is something that has taken my colleagues and certainly myself aback," he says.
Wiley's team has two cases ready for prosecution; they have identified two dozen ISIS members and leaders linked to the persecution of Yazidis. Isis leader Abu Bakr Al-Baghdadi, his deputies and other leaders and members are among the suspects.
While CIJA has been pushing for the establishment of a specialized ISIS tribunal in Northern Iraq, exactly where and how these cases will be heard is tangled up in the complexities of politics and differences between the central government in Baghdad and the Kurdistan Regional Government (KRG) in Erbil.
Many in Iraq want the International Criminal Court (ICC) to pursue ISIS crimes in Iraq, but the country is not a signatory to the Rome Statute that created the ICC, so it has no jurisdiction there.
The KRG's minister of martyrs' affairs, Minister Mahmoud Haji Salih, told CNN by email that his government is looking at several options for high-level prosecutions, including a tribunal set up by the United Nations Security Council.
Failing that, he said, officials would push the Kurdistan Parliament to establish a local court with help from international experts and the central government. Vital to such a court's success would be access to ISIS detainees, Salih said.
But the government in Baghdad "has not helped us in any way," he says.
Iraqi government spokesman Saad al-Hadithi said he was not aware of any such requests for help from the KRG.
He said ISIS members had been captured in areas under Iraqi government control and their trials should take place under the authority of the federal government.
On September 21 the UN Security Council unanimously adopted resolution 2379, agreeing to send investigators to Iraq to collect and preserve evidence of crimes that could amount to war crimes, crimes against humanity and genocide.
From Mosul, Nadia and her sister were taken to Syria, where Nadia spent a year being moved between Homs, Tabqa, ISIS' self-declared capital, Raqqa, and Deir Ezzor.
Nadia says her sister Amira killed herself after she was forced to "marry" an ISIS fighter.
After her suicide, Nadia says she was determined to get away. She and some other Yazidi captives tied their headscarves together to form a rope they used to climb out of the house where they were being held captive.
But their freedom was fleeting. They were caught after hours on the run, and forced to return to their "prison" the following day.
Sold to a Syrian ISIS fighter, Nadia says she became pregnant; to avoid having a child conceived by rape, she threw herself from walls and took pills to terminate the pregnancy.
"I did not want it, what else could I do?" she asks.
Her Syrian captor disappeared, and she was later told he had died. "I was happy when they told me," he says.
Taken by a Saudi ISIS fighter and his Syrian wife, she was later sold again, this time to an Iraqi who took her back to Mosul.
The western Mosul house she was held in was hit by the fighting earlier this year, leaving her wounded: Her legs are badly burned, her arms covered in shrapnel scars.
In July, an airstrike hit the house; several ISIS fighters and a Yazidi slave were killed, she says. Spotting a chance to escape, she darted out of the house and ran until she reached an Iraqi Army unit.
"No matter what happens I will never forget... How can I forget?" Nadia said.
In addition to working with CIJA, the Kurdistan Regional Government says it has prepared more than 2,000 case files, many filled with victims stories like Nadia's.
ISIS-linked trials, under the country's terrorism laws, have been taking place in mobile courts around Mosul and at the country's central criminal court.
Iraq executed 42 men convicted of links to ISIS and al-Qaeda in a single day last month, according to the UN.
But rights groups like Amnesty International have voiced concerns over the potential for unfair trials, saying that Iraq's criminal justice system is "deeply flawed."
In 2005, Wiley was one of the US-appointed advisers to the Iraqi High Tribunal, set up to try Saddam Hussein and his regime. The trial was marred by political interference from the Shiite-dominated government, according to Wiley, who believes it missed a chance to contribute to national reconciliation.
Wiley says Iraq should make a clear distinction between ISIS members and the Sunni Arab tribesmen who fought alongside ISIS in 2014 because they felt marginalized and discriminated against by the government.
Failure to do so could contribute to the emergence of another ISIS-type group that exploits sectarian divisions in Iraq, he warns.
"There is a danger ... that if prosecutions are focused only on the murders of Shia, Yazidi, Christians and so forth, we are going to ... reinforce the sectarian narrative that has been so damaging to Iraq since 2003."
CIJA has also been investigating alleged war crimes in Syria, where the team has built several cases against members of the Assad regime.
The Syrian regime has consistently denied allegations it has carried out war crimes, such as chemical weapons attacks, against its own people. Bashar al-Assad's government says it has been fighting "terrorists" in the country since 2011.
Its work in Syria continues, funded by international donors. But Wiley says progress in Iraq is under threat if western donors are unwilling to add to the $7 million already spent there.
"It's up to them if we carry on in Iraq," he said. "The Iraqis cannot do it on their own ... for various reasons." Any involvement by the UN, he says, "would be a very positive development."
Justice, Shaima says, is a "difficult concept" and something she frequently finds herself trying to explain to the victims of ISIS' horrors.
She tells them it is about making sure perpetrators are not allowed to get away with the atrocities ISIS has committed, and sending a message to those thinking of joining the group.
Nadia has a simpler view of what justice is about for her: "Killing them all. It's best," she says, her voice soft and childlike. "Why keep them alive? If they are captured, they will not be honest about what they did."
As for her own future, Nadia has an idea how she wants to pass the rest of her days. She may not be able to remember how many younger siblings she has, or their names and ages, but she wants to spend her time keeping them safe.
Asked if she wants to have a family of her own someday, she says simply: "Why, after all that I have been through?"