Warning: This article contains images and descriptions of extreme violence.

Human Rights Watch released a report last week detailing — with horrific precision — a single massacre carried out by the Islamic State militant group (also known as ISIS or ISIL) in which more than 600 people were thought to have been executed. Witnesses say an ISIS militant filmed the atrocity.

While Human Rights Watch says no footage of that massacre has been released, ISIS has bragged about previous atrocities, releasing nauseating videos of mass executions.

In explicit detail, one video released by the group, and seen by Business Insider, appears to shows the executions of at least 160 men and boys.

Militants posted images in June and said the victims were Iraqi soldiers, though most appear to be young men and boys in civilian clothing. ISIS said it killed 1,700 Iraqi prisoners of war, though human rights groups say the figure should be treated with some skepticism.

A subsequent investigation by Human Rights Watch, including interviews with witnesses and analysis of the footage, has revealed that between 560 and 770 people were killed in the massacres, which occurred in the Iraqi city of Tikrit.

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The footage shows the victims crammed into trucks for transport and bound and blindfolded lying facedown. In severe distress, some of the men plead with their captors, while others appear despondent.

In one mass execution, at least 30 men are forced to march at gunpoint to a shallow pit in a desert landscape. They are then made to lie face down, with their hands behind their head.

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A man with an automatic weapon held casually in one hand then walks down the row of prisoners, firing bullets into each person one-by-one.

The victims' bodies spasm with each shot; several are seen writhing after their injuries. The shooter, accompanied by a fellow militant holding an ISIS flag, then takes a second pass, firing more bullets into the prisoners.

In the image below, Human Rights Watch says it appears the mounds of dirt to the right are covering a mass grave of previously executed prisoners. A partially buried body appears to be visible in the lower right of the image.

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In another execution depicted in the video, three men are forced to kneel with their backs to a gunman on a large slab of concrete. Scores of other bodies are visible in the background.

The kneeling men have their hands tied and their faces blindfolded. Each is shot in the head by a masked man with an automatic weapon.

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In the final section of footage, at least 15 men are forced to march in single file with their heads down and their hands on one another’s waists. One man appears to argue with a gunman, who then fires a shot above his head, prompting the captive to fall back into line.