The head of the UN special probe into Islamic State group crimes has called for trials like those at Nuremberg of Nazi leaders to ensure the jihadists' victims are heard and their ideology "debunked".
For a year, British lawyer Karim Khan has travelled around Iraq with a team of almost 80 people to gather evidence and witness testimony for the UN body known as UNITAD.
"It's a mountain to climb", the human rights specialist told AFP, as the investigative team works to analyse up to 12,000 bodies from more than 200 mass graves, 600,000 videos of IS crimes and 15,000 pages from the group's bureaucracy.
- Crucifixions, slavery, beheadings -
Five years ago, when their self-proclaimed "caliphate" spanned territory the size of the United Kingdom, the jihadists imposed their brutal rule over seven million people across Iraq and Syria with administrations, schools, child soldiers, a severe interpretation of Islam and medieval punishments.
Minority groups considered by IS to be "heretics" or "satanists" were killed by the thousands, tortured or enslaved.
IS "wasn't some kind of guerilla warfare or a mobile rebel group... that's one aspect that is unusual" for international justice, Khan said from the ultra-secure UNITAD headquarters in Baghdad.
"There was no taboo" for IS, Khan said.
"Who could have thought in the 21st century we would see crucifixion or burning a human alive in a cage, slavery, sexual slavery, throwing people off buildings, beheadings".
And all this captured "with a TV camera".
Despite the horror, these crimes "are not new," he said. "What is new perhaps with IS, is that the ideology fuels the criminal group in the same way that fascism fuelled the criminal pogroms of Hitler".
- 'Separating the poison'-
Nazi leaders were put on trial at the 1945-1946 international military tribunal at Nuremberg, Germany for the murder of around six million Jews during World War II.
Today almost every day Iraqis are sentenced, often to death, but the victims are not present at the trials and the only charge brought is belonging to IS.
But Khan said trials where evidence and testimony are exposed to everyone are the only way to turn the page.
After IS, "Iraq and humanity requires its Nuremberg moment", he said.
Because of Nuremberg, "nobody could be taken seriously if they would espouse the principles of Mein Kampf (written by Adolf Hitler). In fact alarms bells in the public conscience would be aroused if anybody thought the principles of fascism were an alternative political philosophy", he added.
Nuremberg also "separated the poison of fascism from the German people", according to Khan.
"It was one of the principles of Nuremberg that there is no collective guilt", but individuals held responsible, and condemned.
A fair trial for IS "can also contribute to separating the poison of IS from the Sunni community", a minority group in Iraq where two thirds of the population is Shiite, Khan said.
And where "Nuremberg also educated Germany (and) Europe", an IS trial would have an "educative effect, not only in the region, but in other parts of the world where communities may be vulnerable to the lies and propaganda of IS".
"That ideology can be debunked, so people that are watching... can realise a self-evident truth, that it was the most un-Islamic state that we have seen", Khan added.
UNITAD is working to establish if IS actions constitute crimes against humanity, war crimes or genocide, the most serious crimes in international law.
"You will see in the next two months that we are feeding into some prosecutions that are already taking place in some states," he added.
- Universal jurisdiction -
UNITAD "will build our own cases also" that will permit states which like Germany have universal jurisdiction to deal with crimes regardless of where they were committed and the nationality of the perpetrators and victims.
Some trials are already in motion, notably in France for attacks claimed by IS and in Munich where a German woman has been charged with having left a young Yazidi girl "purchased" at an IS slave market to die of thirst.
But, Khan said, "Iraq is the primary intended recipient of our evidence, of our information".
Iraq has already tried thousands of its own nationals arrested on home soil for joining IS and has sentenced hundreds to death, whether they fought for the group or not.
"The forum is not so important", Khan said, as the possibility of an international tribunal has been raised by some but seems unlikely in the near future.
What is essential is that "the victims have the right to have their voice heard".