The story comes from The Telegraph.
Massacre in Syria as al-Qaeda fighters flee rebel backlash
Forced from Aleppo by their former allies, the Islamic State of Iraq and al-Sham committed a final atrocity
The pictures look like those of victims of the regime's many massacres. Corpses lie huddled in the courtyard, hands tied behind their backs, bleeding from the head.
The stories of the survivors are similar, too. "I was held in solitary confinement for 13 days," said one, Milad Shehabi. "They tied me up and stamped on me with their military boots. They said, 'You dogs, you media people, your revolution is a revolution of thieves'."
Another man described the executions he had witnessed, carried out without any form of due process, for the most capricious of reasons.
"They tortured two Kurds," he said. "They said to them, 'Are you related to the PKK (the Kurdish separatist group)'.
"They didn't say yes but they killed them anyway. Another Kurd was just walking by. They put the gun to his head and killed him."
But the people responsible were themselves revolutionaries, so the survivors said. The revolution in Syria, already racked by division, has now created a civil war within a civil war, and the atrocities alleged to have been committed against their own supposed fellow revolutionaries by ISIS, the Islamic State of Iraq and al-Sham, now resemble those that began the war in the first place.
The backlash against ISIS, a branch of al-Qaeda whose work in Syria has even been disavowed by the international organisation's head, Ayman al-Zawahiri, had been rumbling for some time before it blew up at the beginning of the month. Other revolutionary militias swept into towns controlled by ISIS last weekend, expelling the group from some of its strongholds.
Within two days, fighting was raging in both Raqqa, the only provincial capital completely under rebel sway, and Aleppo. By Wednesday, ISIS had been forced to retreat from the latter, and found itself on the defensive in a number of Aleppo province's smaller towns.
Given ISIS's reputation for brutality - its history of publicly decapitating enemies, whipping wrong-doers and even, on one occasion, shooting a 14-year-old boy dead in front of his parents for taking the Prophet Mohammed's name in vain - no-one was particularly surprised to find what their balaclava'ed fighters left behind.
It was known they had a prison under one of their bases in Aleppo. It is where a sizeable number of western journalists and aid workers seized by the group for unspecified purposes - no ransoms have been sought - have been held.
There was no sign of these. Activists contacted from outside of the country said they thought the westerners had been taken away with the retreating ISIS forces. But 300 local prisoners were left behind.
Another 50, by one count apparently matching the videos, were killed before they left.
Some of the survivors' accounts have now been recorded for the cameras. Mr Shehabi, a well-known "citizen journalist" working from an office in the rebel-held Aleppo suburb of Masaket Hanano, scene of heavy fighting in late summer 2012, had been seized from his office by the group on December 26.
He described his treatment as "worse than Air Force Intelligence" - the most notorious of the Assad regime's military units. He said the prison, a former children's hospital, had 11 rooms for prisoners, each holding 25-30 men. When the rebellion against ISIS started, he said, a fighter came to each cell.
"You can't rebel against us," he said. "We are the Islamic State of Iraq, we have been in Iraq for 10 years and they couldn't get rid of us." ISIS grew out of the Iraqi branch of al-Qaeda, which seemed to have been defeated by the American army's "surge" but in fact continues to fight its own war against the Baghdad government.
Other released prisoners identified themselves as members of rival brigades that had fallen foul of ISIS. One of ISIS's most bitter battles had been against the Ahfad al-Rasoul, or Grandsons of the Prophet, Brigade, which challenged ISIS for control of Raqqa and was accused by ISIS of widespread corruption.
"I was stopped by ISIS at a check point," said one man. "They asked me for my identity and I gave them the ID of Ahfad Al-Rasoul. I was arrested and for two months I was kept tied up. They told us, 'You are grandsons of the stain, not of the Prophet'.
"The deputy head of the brigade was imprisoned with us, Wardan Qasoum. He was tortured for five days. They were asking him, 'Did you become an infidel?' He answered no, until finally under the torture and humiliation he said to them, 'Yes, I become an infidel.' Then they killed him."
The attacks on ISIS have the hall-marks of a planned operation, and if not organised by the backers of the non-al-Qaeda rebels, principally now Saudi Arabia, will certainly win their support. The West, too, wants to see a non-al-Qaeda rebel movement that is in some way coherent in place across northern Syria before attending peace talks in Switzerland next week.
But they face the problem that for many Syrian civilians, although ISIS is undoubtedly brutal and Jabhat al-Nusra, the second and officially recognised Syrian al-Qaeda franchise narrowly ideological, accusations of the corruption of the brigades fighting ISIS also ring true.
The leader of the uprising against ISIS is said to be a non-Islamist brigade leader called Jamal Maalouf, from Idlib province. Last November, other rebels in the area told The Sunday Telegraph of their disgust at how he had enriched himself from the war.
In any case, ISIS is not going quietly. An member from Britain - ISIS has the highest dependence on foreign fighters - described last week how it was counter-attacking as far south as Homs, in central Syria.
The anti-ISIS factions were initially reported as having taken Raqqa, but fighting continues, with reports of dead bodies lying in the streets.
The Syrian Observatory of Human Rights, a UK-based monitoring group, puts the total death toll from the fighting at almost 500, including 85 civilians. Fighting on Saturday was reported both in Raqqa and along the Turkish border. Syria continues to break down, piece by piece.