Former international prosecutors said Tuesday they have evidence from a defector proving that the regime of Syrian President Bashar al-Assad has systematically killed and tortured around 11,000 people.
A report by three top investigators commissioned by Qatar, which backs the Syrian rebels, examined thousands of pictures said to have been smuggled out by a former military police photographer.
The report, which was first released in the Guardian and on CNN, shows evidence of starvation, strangulation and beatings, and features pictures of emaciated corpses with livid wounds.
The release came a day before talks were due to begin in Geneva aimed at negotiating an end to Syria's bloody civil war.
Syria denies torturing detainees.
British Foreign Secretary William Hague said the report "offers further evidence of the systematic violence and brutality being visited upon the people of Syria by the Assad regime."
Evidence of "industrial-scale" killing
The report was written by Desmond de Silva, former chief prosecutor of the special court for Sierra Leone; Geoffrey Nice, the former lead prosecutor in the trial of former Yugoslavian president Slobodan Milosevic; and David Crane, who indicted Liberian president Charles Taylor.
It also features testimony from a forensic pathologist, an anthropologist who investigated mass graves in Kosovo and an expert in digital images.
De Silva said the report was the "smoking gun" showing evidence of "industrial-scale" killing by the Syrian regime.
The defector, identified only as "Caesar" for his own safety, presented forensic experts commissioned by a London legal firm representing Qatar with around 55,000 digital images of 11,000 dead detainees since the start of the uprising in Syria in March 2011. The images were on memory sticks.
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He claims the victims all died in captivity before being taken to a military hospital to be photographed.
Starved, mutilated, tortured
"The pictures show over a period of years the systematic murder of detainees by starvation, by torture, the gouging out of eyes, the hideous beating of people, the mutilation of bodies," De Silva told the BBC on Tuesday.
The report says that all but one of the victims were male. Most appeared to be aged between 20 and 40 and a "very significant percentage" showed evidence of starvation.
The defector "informed the inquiry team that there could be as many as fifty bodies a day to photograph which required fifteen to thirty minutes of work per corpse," the report said.
He said the purpose of the photos was firstly to be able to issue death certificates — falsely saying that the victims had died in hospital — and secondly to confirm to the regime that executions had been carried out.
The bodies would then be buried in rural areas.
'He couldn't take it anymore'
The authors of the report said they found the informant and his evidence to be credible after subjecting them to "rigorous scrutiny" and have made their findings available to the United Nations, governments and human rights groups.
The fact that the defector had not claimed to have actually witnessed any of the killings added credibility to his story, they said.
He later escaped from Syria fearing for the safety of his family.
"There came a point a few months ago where he decided that he couldn't take it anymore, so he decided to defect and he left. He could well have gone to Qatar, yes," said De Silva.
Sunni-ruled Qatar was quick to back rebels who rose up in 2011 against the rule of Assad, who is backed by Shia powerhouse Iran.
Crane called the evidence "amazing" and suggested there was a strong case for prosecution.
"Now we have direct evidence of what was happening to people who had disappeared," he explained.
"This is the first provable, direct evidence of what has happened to at least 11,000 human beings who have been tortured and executed and apparently disposed of."