MONTREUX – Behind the closed doors of a hotel room in this Swiss city, away from the commotion of the nearby peace conference on Syria, a man is scrolling through photographs of corpses on his computer screen.

Often naked or covered with rags, the bodies bear traces of different types of torture: laceration, strangulation, electrocution, mutilation. On most of the chests, numbers written with a marker identify the victims. For others, it's a piece of cardboard placed at their feet: "It's the number that's given to the detainees when they're arrested and when they're pronounced dead," explains the man, an opponent to Bashar al-Assad's government named Emadeddin Rachid.

"The numbers follow each other," he says. "It's assembly-line killing."

These pictures, to which Le Monde, has had exclusive access, fed the report made public (see here) ahead of the conference by CNN and The Guardian, and which accuses the Syrian regime of having tortured and killed on an "industrial scale."

The study, commissioned by Qatar and carried out by a British law firm as well as with international law experts, is based on material of a nature and on a scale unprecedented in Syria: 55,000 pictures representing some 11,000 people killed in custody.

How were they verified as authentic? The person who's responsible for this massive leak is none other than the man who took the pictures: a photographer with the Syrian military police who defected in 2013, identified in the report under the name "Caesar."

Emadeddin Rachid is one of the people behind the operation, which landed with a thud at the would-be peace conference and destabilized the Syrian delegation in front of the international press. Aged 48, a former deputy head of the Sharia Faculty at the University of Damascus, he is also one of the leaders of the Syrian National Movement, a moderate Islamist branch represented inside the Syrian National Coalition.

In all likelihood, it's thanks to an old connection between a member of his movement and "Caesar" that contact was established. The man was for a long time in charge of taking pictures of crime or accident scenes, but when the revolution started in 2011, he was entrusted with a whole new task: take photographs of real or supposed opponents who had been tortured to death or executed in cold blood in the government's prisons.

This military census work, as meticulous as it is macabre, was carried out with two goals in mind: First, for the authorities to be able to deliver a death certificate to the families looking for a disappeared brother or father, putting the blame on a "respiratory problem" or a "heart attack;" second, for the torturers to be able to confirm to their superiors that the dirty work was done.

"Killing its opponents is the regime's routine," explains Rachid. "Registring torture is nothing more than the continued pursuit of the routine."

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A pro-Assad rally in Syria in 2011 (Sammy.aw)

At the military hospital where he was assigned, "Caesar" used to receive up to 50 bodies per day. Each of them required between 15 and 30 minutes of work, as four or five pictures were required for the file. The spectacle of the Syrian secret services' savagery was too much for him, and convinced the forensic photographer to join the rebellion.

"Rigorously authenticated"

It took six months to set up the channel to collect the pictures. "A network of anonymous people, including fighters of the Free Syrian Army, risked their lives for this operation," Rachid says. An extra four months were needed to smuggle "Caesar" and his family out of Syria.

The London-based Carter-Ruck law firm then called upon three forensic examiners and three former international prosecutors at the criminal tribunals for Sierra Leone and the former Yugoslavia.

Qatar was well aware that its unequivocal opposition to Bashar al-Assad might jeopardize the credibility of the final report, so it accepted to finance the study on condition that "the evidence is properly and rigorously authenticated," says Cameron Doley, one of the lawyers of the firm.

The experts buried themselves in the flow of pictures and grew convinced that "it was very unlikely" that they could have been forged. During the month of January, they met with "Caesar" on three occasions, asking question after question, eventually confirming that the man is indeed who he claims to be.

Although he supported the anti-Assad uprising, "Caesar" "gave an honest account of his experiences," according to the report, adding that he never pretended to have witnessed the executions.

Speaking to the BBC, Sir Desmond de Silva, who led the inquiry team, rejected any claims of interference from Doha. Just because Qatar has "a vested interest does not mean the evidence is untrue," he said. "And we were meticulous in the way we went about our work and indeed we did so in that knowledge that in Syria, there are many conflicts and there are many interests both national and international."

Emadeddin Rachid showed Le Monde a scanned copy of a death file made of "Caesar's" pictures. The form carried the heading of the "Syrian Arab Republic, General Command of the army" and on the postmortem pictures, the seal "Military Police" is affixed.

Le Monde was also able to see several pictures of a warehouse, turned into a mass grave, with some 15 emaciated corpses scattered on the floor. "This is the garage of the military hospital of Mezzeh," says Rachid. "That's where they throw the corpses when the morgue is full. All these bodies, all skin and bones: It inevitably makes you think about Nazi concentration camps."

"Caesar" and his companions now dream of the day when they can hand the photographs to an international court. "If justice doesn't take over and doesn't do what needs to be done after such massacres, then you can bet that there will be counter-massacres," Rachid warns.