The UN and independent human rights groups have documented abuses by both Bashar al-Assad's government and rebels, but experts say this evidence is more detailed and on a far larger scale than anything else that has yet emerged from the 34-month crisis.
The three lawyers interviewed the source, a military policeman who worked secretly with a Syrian opposition group and later defected and fled the country. In three sessions in the last 10 days they found him credible and truthful and his account "most compelling".
They put all evidence under rigorous scrutiny, says their report, which has been obtained by the Guardian and CNN. (Read here)
The authors are Sir Desmond de Silva QC, former chief prosecutor of the special court for Sierra Leone, Sir Geoffrey Nice QC, the former lead prosecutor of former Yugoslavian president Slobodan Milosevic, and Professor David Crane, who indicted President Charles Taylor of Liberia at the Sierra Leone court.
The defector, who for security reasons is identified only as Caesar, was a photographer with the Syrian military police. He smuggled the images out of the country on memory sticks to a contact in the Syrian National Movement, which is supported by the Gulf state of Qatar. Qatar, which has financed and armed rebel groups, has called for the overthrow of Assad and demanded his prosecution