Months after Mosul was reclaimed from the Islamic State, the brutal acts committed during the militant group's reign over the major Iraqi city are still coming into focus. That picture grew clearer Thursday, as two U.N. human rights agencies released a report on atrocities committed during ISIS' final months in power.

In a stroke of apparent understatement, the Office of the U.N. High Commissioner for Human Rights and the U.N. Assistance Mission for Iraq concluded that their full collection of eyewitness accounts "strongly suggests that international crimes may have been perpetrated in Iraq by ISIL," referring to the extremist group by another abbreviation.

"During the course of the operation to retake Mosul City thousands of civilians were subjected to shocking human rights abuses and clear violations of international humanitarian law," said U.N. human rights chief Zeid Ra'ad Al Hussein. "The execution-style killing of civilians, the suffering inflicted on families, and the wanton destruction of property can never be tolerated in any armed conflict, and those responsible must answer for their heinous crimes."

Over a span of 49 pages, the two organizations listed those crimes in clinical detail: abductions, mass killings, deployment of child soldiers, use of improvised explosive devices. From October 2016, when the Iraqi Security Forces launched their operation to retake Mosul, to the moment the country declared victory in the city in July 2017, the report says at least 2,521 civilians were killed in and around Mosul — including 741 who were executed.

And, the U.N. adds, "these figures should be considered an absolute minimum."

"During a two-week period (17-29 October 2016) alone, ISIL killed around 550 civilians and former ISF members" in the areas in the approach to Mosul, according to the report. "In one single instance, on 26 October, ISIL allegedly shot and killed 190 former ISF personnel in the al-Ghazlani military base in Mosul."

This mass killing was just one of many reported to the U.N., which often claimed dozens of victims at a time. All told, the report says, "since June 2014, at least 74 mass graves were discovered in areas previously held by ISIL in Iraq" — some of which "are estimated to contain the remains of up to thousands of victims."

As the fight for Mosul crept into the bounds of the city itself, with bullets exchanged street by street, ISIS militants reportedly went door to door, abducting the young sons of families they found there. These children — including a unit the group called "Cubs of the Caliphate" — would then be strapped with explosive vests or belts and deployed against security forces.

"Children continued to be seen in ISIL propaganda published on social media and websites during the third phase of the operation," the report notes. "Numerous images published by the group showed a number of children who were claimed to be members of the 'Cubs of the Caliphate', carrying out military drills with weapons."

A month before Iraq claimed victory in the city, one such "cub" reportedly attempted to blow himself up at the gate of an Iraqi police camp and, after refusing warnings to stop, was shot dead before he was able to detonate his vest.

For their part, Iraqi forces were not entirely free of blame. The report listed a number of incidents of unlawful killings, forced evictions and torture apparently committed by the ISF, many of which were videotaped. Airstrikes also resulted in the deaths of more than 1,000 civilians over the final months of the campaign to take Mosul, according to the U.N. agencies.

But the U.N. groups reserved the harshest of their condemnations for ISIS, or Daesh as it's also known.

"Daesh's reign of terror has spared no one," Hussein said in his statement, "inflicting untold suffering on unarmed residents whose only guilt is that they lived in the areas under ISIL's control."

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