All wars are vicious, but the civil war in Syria seems every day to set new standards for brutality. As the fighting rages in its third year, increasing numbers of atrocities are committed by soldiers and fighters from forces loyal to the regime of President Bashar Assad, as well as armed rebels and Islamic militants from the numerous, loosely aligned groups opposing Assad.

The violence is frequently sectarian in nature, with fighters claiming they act in defense of their faith, be it Sunni, Alawite, Shiite or any of the other sects that contribute to Syria’s religious landscape.

The perpetrators of atrocities themselves often use digital cameras or smartphones to photograph or film their acts of torture and murder, uploading the images to the Internet. These images and videos are used for propaganda, and their authenticity is often impossible to verify. It is very rare that a group of fighters from either side gives a professional photojournalist from a country outside Syria full and unfettered access to chronicle an atrocity as it unfolds. The images above are products of that access.

What follows is a harrowing series of photographs of Islamic militants publicly executing, by decapitation, a young Syrian in the town of Keferghan, near Aleppo, on August 31, 2013.

Because of the danger in reporting inside Syria, it was not possible to confirm the identity or political affiliation of the victim. Nor are we certain about the motivation of his killers. One eyewitness who lives in the area and was contacted by TIME a week after the beheadings said that the executioners were from ISIS, an Al-Qaeda franchise operating in Syria and Iraq.

TIME obtained the images exclusively from a photographer who was recently in Syria. This decapitation was the last of four executions he documented that day. TIME has agreed not to publish the photographer’s name, to protect him from repercussions when he returns to Syria.

What follows is an edited account of his experience: