Story highlights Execution carried out by young men, some appearing to be teenagers

ISIS captured historic city of Palmyra in mid-May

2,000-year-old city a UNESCO World Heritage site

Atlanta (CNN) A just-released video from ISIS shows militants executing 25 captives in the ruins of a Roman amphitheater in the ancient Syrian city of Palmyra.

The graphic video begins with images of the Quran, followed by gruesome scenes showing what appears to be the aftermath of fighting, with bodies in the street. Then it shows captives, heads down and dressed in dark fatigues, as they are herded single-file onto the stage of an ancient arena. A row of young men, many appearing to be in their teens, stands behind the prisoners while an older man reads a document to the assembled crowd.

Periodically, the video shows men seated on the crumbling stone seats. Some are waving ISIS flags, but others appear distressed, hiding their faces and even crying.

When the cue is given, the young militants shoot the assembled captives all at once. The scene includes an editing trick which runs the film in reverse, making the deceased appear to rise and get shot again.

The video shows a row of young men standing behind the prisoners while an older man reads a document to the assembled crowd.

CNN could not independently confirm the authenticity of the recording.