LIKE many refugees, Caesar had terrible tales to tell when he slipped out of Syria in August 2013. Less typically he also had pictures to document his story: 53,275 of them, to be exact. From shortly after the eruption of civil war in the spring of 2011 until his flight, Caesar (a pseudonym) worked for government security forces as a forensic photographer. The photos he took, and surreptitiously copied, were of thousands of corpses. Some were of fallen soldiers or war victims, but most were of young men who had spent their last days in the dungeons and torture chambers of the Syrian regime.

The pictures show that thousands of prisoners died of fatal neglect, vicious abuse or straightforward murder by beatings or gunshots. Some images reveal groups of naked, emaciated bodies, each tagged with numbers, splayed on the dirt floor of a hospital garage. But because Caesar entrusted his grisly trove to Syrian opposition groups that publicised the atrocities via a London law firm hired by the government of Qatar, which is hostile to the Syrian regime, some have questioned its authenticity. “You can bring photographs from anyone and say this is torture,” said Bashar al-Assad, Syria’s president, in an interview earlier this year. “So it’s all allegations without evidence.”

Such doubts should now be laid to rest. Following a six-month investigation that included dozens of interviews with former prisoners, defectors who had worked in Syrian military hospitals or intelligence agencies, forensic experts and families of the disappeared, Human Rights Watch (HRW), an independent watchdog group, says it is satisfied that the photos are indeed genuine. In a report published on December 16th it says that Caesar’s work suggests that Syrian officials should be tried for crimes against humanity.

Among other confirmatory evidence, HRW researchers traced the cases of 27 individual victims back to their families, matching Caesar’s pictures with family photos and tallying the dates and places of their arrest against details recorded by Caesar. Forensic analysis of pictures revealed not only telltale signs of starvation, severe skin infections and bruising, but of “chronic venous insufficiency in the lower extremities”, the medical term for legs swollen by being forced to stand for long periods, a detail that confirms former prisoners’ stories of crowding so severe that being taken out for torture was seen as a welcome chance for fresh air. Numerous families also testified to paying government officials extortionate bribes for news of imprisoned relatives, only to discover from the pictures that they had long been dead.

Judging from the picture sets, which often include several of the same corpse, it appears that Caesar photographed more than 6,000 dead prisoners. These represent most, but not all, of those he witnessed during his shifts at two military hospitals in the Syrian capital, Damascus, over a 27-month period. But Syria’s war has lasted more than twice that long. Damascus, where there are other military hospitals that similarly ‘processed’ prisoners’ corpses, holds just a quarter of the country’s people. The full scale of the carnage in Mr Assad’s prisons may never be known.