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.
CNN could not independently confirm the authenticity of the recording.
The video is approximately10 minutes long and is undated. According to the British-based Syrian Observatory for Human Rights, which received a copy of the video, the execution occurred on May 27, shortly after ISIS fighters took over the ancient city.
The video also shows militants blowing up the notorious Tadmur prison, which was used to house political prisoners in Syria, during the regimes of President Bashar al-Assad and his father, President Hafez al-Assad. A 2001 report from Amnesty International called conditions at the facility "brutal and dehumanizing."
The ISIS video shows prison cells and boxes of files, followed by ISIS fighters rigging the grounds with explosives. A masked militant is shown pushing a detonator. Then several images of the explosion are shown, once again featuring special editing effects.
UNESCO site in jeopardy
The modern city of Tadmur, where the prison was located, sits just a few hundred yards from the ancient colonnades and temples of Palmyra. It is a UNESCO World Heritage site that dates back some 2,000 years.
An oasis known as "the bride of the desert," Palmyra sat on an important trade route that linked the Roman Empire with Persia, India and China. Its crumbling monuments reflect a mix of classical, Iranian and Arab architectural styles.
Since ISIS seized the city in May, it has destroyed several historic sites, including the tomb of Muhammed bin Ali, a descendent of Ali bin Abi Taleb, the Prophet Mohammed's cousin. It has also smashed artefacts and statues -- among them the Allat God statue, an important ancient object depicting a lion catching a deer between its feet.
Other temples, monuments and historic buildings have been mined, but it's not clear whether the explosives were put in place for the purpose of destruction or preventing the advance of Syrian government troops.
The U.N.'s cultural organization has accused ISIS of "cultural cleansing" as a part of a global propaganda campaign, aimed at recruiting foreign fighters and dismantling the fabric of societies across the Middle East.
ISIS emerged from Syria's civil war
Tadmur is also home to tens of thousands of people. After militants took control of the city, it went door- to-door looking for Syrian government soldiers. ISIS has executed scores of residents -- including one group that allegedly contained at least 11 children, along with hundreds captive fighters.
According to the nonprofit news organization Syria Direct, the capture of Palmyra/Tadmur wasn't just about seizing the prison or controlling a symbol of historic importance, but also about securing an area the Syrian government considers to be "the first line of defense" against ISIS attacks from the eastern regions.
The city is a point of defense for surrounding oil and gas fields, including the Shaer gas fields, which are important to Syria's electricity sector. Control of the area means ISIS has a clean line of access to Damascus and to Homs.
Civil war broke out in Syria four years ago. During the fight against al-Assad, a variety of groups have emerged, including ISIS. It now controls more than half the country, effectively running parts of ten of Syria's 14 provinces.
From Syria, the group has spread to neighboring Iraq, where it has seized the key cities of Ramadi and Mosul.