Iraqi authorities will begin identifying the remains of 141 people exhumed from mass graves in the Yazidi region of Sinjar, the head of Baghdad’s forensic office said Thursday.

“The remains will first be examined, and then DNA samples will be taken to compare with samples gathered from families,” Zaid al-Yousef told AFP.

The efforts are part of an investigation by the Iraqi government and a special United Nations team to collect evidence of crimes committed by the Islamic State group.

IS swept across swathes of Iraq in 2014, including the Sinjar region where the Yazidi minority was long based.

The Kurdish-speaking Yazidis follow an ancient religion, but IS considered them “apostates”.

The jihadists forced thousands of Yazidi women and girls to be “sex slaves”, recruited boys to fight, and executed Yazidi men en masse in what the UN has said could amount to genocide.

The UN began its joint probe last year, exhuming the first mass graves of IS victims around the town of Kojo in Sinjar in March.

It said last month that 12 of 16 identified grave sites around Kojo had been exhumed.

But Yousef said the next phase of identifying the victims would be a fraught process.

“We took around 1,280 samples from families in Sinjar, but the problem is that for a lot of them, there’s just a single survivor and the rest are all missing,” he said.

“If we compare it with other terrorist attacks, we would find three, four, or five survivors for every missing person. But here, we have three, four, or five missing people for a single survivor,” Yousef added.

He said the identification process would also be impacted by the rate of intermarriage among Yazidis, who very rarely wed outside the community.

That insularity is part of what made IS’s 2014 sweep so scarring, with many Yazidi women who were abducted and raped by IS initially excommunicated.

Yazidi spiritual leader Baba Sheikh issued a decision the following year welcoming those women back home, but the fate of children born of those rapes remains unresolved and controversial.

Many Yazidi women who were kidnapped by IS have escaped in recent years, and dozens more fled to safety this year as IS’s “caliphate” crumbled in Syria, but several thousand remain missing.