– More than 200 mass graves holding as many as 12,000 bodies have been found in areas of Iraq that were once controlled by ISIS, United Nations investigators said on Tuesday.

The 202 graves verified by investigators are concentrated in northern and western Iraq, areas that ISIS controlled from 2014 to 2017. A joint report by the U.N. mission to Iraq and the Office of the High Commissioner for Human Rights called the sites the militants’ “legacy of terror.”

The deaths occurred in what the United Nations has labeled systematic and widespread violence, a campaign that “may amount to war crimes, crimes against humanity, and possible genocide.”

“ISIL’s horrific crimes in Iraq have left the headlines, but the trauma of the victims’ families endures, with thousands of women, men and children still unaccounted for,” Michelle Bachelet, the U.N. human rights commissioner, said, using another acronym for ISIS. “These graves contain the remains of those mercilessly killed for not conforming to ISIL’s twisted ideology and rule.”

Iraqi authorities have so far exhumed 1,258 bodies from 28 sites, the U.N. said, making it difficult to establish exactly how many bodies are contained in the graves documented in the report. The international organization urged officials to identify the victims quickly and to return the bodies to relatives. Investigators said the smallest site, in west Mosul, contained eight bodies; the biggest is believed to be the Khasfa Sinkhole near Mosul, which may contain thousands.

During its three-year rule, ISIS terrorized residents, conducting well-publicized executions of people targeted for government ties, sexual orientation and other reasons. The militants also went after members of ethnic and religious minorities, including Christians and Yazidis.

U.N. officials in Iraq had previously estimated that 30,000 civilians were killed by ISIS, a number that “should be considered an absolute minimum,” investigators said. How many of those victims were buried in mass graves was unclear, but the report suggested that it was a large proportion of the total.

The United Nations said it was likely that more mass graves would be discovered.

It also noted that Iraqi bureaucracy made it difficult for people to find missing relatives because information was not held in a centralized way.

“Their families have the right to know what happened to their loved ones,” Bachelet said.

Investigators recommended setting up a nationwide databank, similar to those set up in Bosnia and Rwanda.