ISIL made headlines around the world in 2014, when its fighters pushed out Iraqi soldiers and took control of cities like Mosul and Sinjar.

And in what became known as the Sinjar massacre, thousands of Yazidi men, women and children were killed. In response to ISIL’s rapid territorial gains, the United States and its allies formed a coalition to fight the armed group.

They launched air strikes on ISIL hideouts, its fighters and convoys. In October last year, a US military operation in northwest Syria resulted in the death of ISIL’s leader Abu Bakr al-Baghdadi.

In 2018, the United Nations secretary-general, Antonio Guterres, announced the appointment of a special adviser to support efforts to investigate ISIL’s crimes. As the head of the UN investigative team, the special adviser’s mandate is to collect, preserve and store evidence.

But, how are the investigations conducted? And will they bring justice to the countless victims in Iraq and Syria?

The UN special adviser, Karim Khan, talks to al Jazeera about his efforts to investigate ISIL’s crimes.

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