A team of international investigators funded by the British government is compiling evidence against Islamic State fighters carrying out atrocities in Iraq and Syria, the BBC has learned.
They have spent months preparing up to 400 prosecution-ready files on the group’s senior leaders and fighters.
Internal IS documents acquired by the team show “command responsibility” for numerous atrocities.
The group has taken over swathes of Iraq and Syria in recent months.
Working in strict secrecy, the team of experts has spent all year actively investigating commanders, emirs and provincial governors for war crimes and crimes against humanity.
The atrocities alleged to have been committed by those in their ranks include kidnappings, beheadings, crucifixions, torture and summary executions.
On Wednesday, President Barack Obama vowed to destroy the group after it released a video showing the beheading of American journalist Steven Sotloff.
The group has carried out several more beheadings, including that of another US journalist, James Foley, and Kurdish fighters.
Mr Obama’s comments came as campaign group Human Rights Watch said it had uncovered new sites of Islamic State mass killings in the Iraqi city of Tikrit.
The militants are believed to have murdered more than 500 Iraqi soldiers after taking over a large Iraqi army base in June.
The highly experienced war crimes investigators, working out of a nondescript headquarters in a European city, have avoided all publicity until now.
But in an interview with the BBC, they revealed that they were being funded by the UK Government to the tune of $70,000 (£43,000) a month and employ sources both in Syria and in the neighbouring countries.
“We would like to see the murderers of James Foley brought to justice,” the chief investigator told the BBC. “Having said that, those particular offenders and others like them are not the focus of our investigations.”
“What we are after are the highest-level members of IS because these individuals are just as responsible for the countless murders as those men who kill with their own hands,” he added.
“Indeed, those leaders are more responsible because they don’t kill one person or two people. They are responsible for all the killings.”
The team has boxes of evidence smuggled out of Syria, consisting of documents, memory sticks and witness testimonies.
They even have the actual minutes of an Islamic State provincial meeting that goes into such detail as forbidding their guards in Aleppo province to sleep in after 07:30.
The investigators have managed to build up an intricate picture of how IS is run, with Abu Bakr al-Baghdadi, the self-appointed caliph, at the top.
Directly beneath him are four advisory councils: Sharia (Islamic Law), Shura, Military and Security, with the latter two being the most powerful.
This one-plus-four structure is then duplicated down the chain of command, right down to local level.
