The leader of the UN special probe into "Islamic State of Iraq and Syria" (ISIS) crimes has called for trials like those of Nazi leaders at Nuremberg to ensure the extremists' victims are heard and their ideology is "debunked".
For a year, British lawyer Karim Khan has travelled around Iraq with a team of almost 80 colleagues to gather evidence and witness testimony for the UN body known as UNITAD.
"It is a mountain to climb," he said, as the investigative team works to analyse up to 12,000 bodies from more than 200 mass graves, 600,000 videos of ISIS crimes and 15,000 pages from the group's bureaucracy.
'No taboo' for ISIS brutality
Five years ago, when ISIS controlled swaths of territory in Iraq and Syria, the group imposed its brutal rule over seven million subjects with its own administrations, schools, child soldiers, a severe interpretation of Islam and mediaeval punishments.
Minority groups considered by ISIS to be "heretics" or "satanists" suffered death by the thousands, torture or enslavement.
ISIS "wasn't some kind of guerrilla 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 ISIS, he 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 of it was captured "with a TV camera".
Despite the horror, these crimes "are not new", he said. "What is new perhaps with ISIS, is that the ideology fuels the criminal group in the same way that fascism fuelled the criminal pogroms of [Nazi German leader Adolf] Hitler."
Debunking ISIS ideology
Nazi leaders were put on trial at the 1945-1946 international military tribunal in Nuremberg, Germany, for the murder of about six million Jews during World War II.
Today almost every day Iraqi courts sentence captured ISIS members, often to death, but the victims are not present at the trials and the only charge brought is belonging to ISIS.
Only trials where evidence and testimony are accessible to everyone can turn the page, said Khan.
Because of Nuremberg, "nobody could be taken seriously if they would espouse the principles of 'Mein Kampf'," he said, referring to the autobiographical manifesto written by 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 said.
Nuremberg "separated the poison of fascism from the German people", according to Khan.
After ISIS, "Iraq and humanity require its [ISIS's] Nuremberg moment", he said.
A fair trial for ISIS "can also contribute to separating the poison of ISIS from the Sunni community", a minority group in Iraq, where two-thirds of the population is Shia, Khan said.
An ISIS 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 ISIS".
"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 said.
Prosecuting ISIS everywhere
UNITAD is working to establish if ISIS 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," Khan said.
Some trials are already in motion, notably in France, for attacks claimed by ISIS, and in Munich, Germany, where a German woman has been charged with leaving a young Yazidi girl "purchased" at an ISIS slave market to die of thirst.
But "Iraq is the primary intended recipient of our evidence, of our information," said Khan.
Iraq has already tried thousands of its own nationals arrested on home soil for joining ISIS 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".