As Islamic State retreats the true scale of its atrocities is becoming apparent. Associated Press collated existing documents and testimonies to produce the fullest picture yet – but activists say that thousands more victims buried in shallow mass graves are yet to be discovered.

The agency says it has pinpointed the exact location of 72 Islamic State (IS, formerly ISIS/ISIL) mass graves – 17 of those in Syria, the rest in Iraq – which contain anything from at least 5,200 to over 15,000 victims.

The information came from AllSource, a satellite intelligence firm that has matched photos from space with eyewitness accounts, aid groups such as Yazda, which are recording the systematic slaughter, and often IS itself, which has boasted about killing hundreds of 'infidels' and 'traitors' in its own regularly-broadcast videos.

"They are beheading them, shooting them, running them over in cars, all kinds of killing techniques, and they don't even try to hide it," said Sirwan Jalal, who has been appointed by Iraqi Kurds to investigate the mass burials.

The biggest documented massacre was committed in Camp Speicher in Tikrit in June 2014, when Islamic State gunned down between 1,000 and 1,700 unarmed Shiite Iraqi Air Force recruits, forcing them to shout slogans as they lay down, waiting to be executed.

Thirty-six of the perpetrators were hanged for the war crime earlier this month.

But while that location was well known, another massacre happened near Ramadi just two days earlier. AllSource looked for images of disturbed earth in the city in the northwest of Iraq – big enough to be noticeable from a satellite image – that tallied with accounts given by survivors to Human Rights Watch.

This was a testimony by a man only known as A.S., who was singled out for being a Shiite and put in a line in which each man had to shout out his number: "I was number 43. I heard them say '615,' and then one ISIS guy said, 'We're going to eat well tonight.' A man behind us asked, 'Are you ready?' Another person answered 'Yes,' and began shooting at us with a machine-gun."

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A.S. told HRW that he escaped by playing dead and then sneaking out at night, among about 15 others, a common tactic among the few survivors of such large-scale massacres.

With war still ongoing, in places such as Hardan, a Kurdish area, the authorities have merely roped off the mass graves, and say there are currently no resources to excavate and document the dead.

As the bodies continue to decay, and the wind blows away the earth, revealing the still-clothed bones, locals are served a daily reminder of the horrors.

"I have lots of people I know there. Mostly friends and neighbors," Arkan Qassem, who lives in a village outside Hardan, told AP. "It's very difficult to look at them every day."

With IS counting different sects – such as Shias and Yazidis – different ethnicities – such as Kurds – and even Sunni tribes as their enemies, there are estimated to be “hundreds” of mass graves that will take years to be fully mapped, and their victims to be given a proper burial.

“This is a drop in an ocean of mass graves expected to be discovered in the future in Syria,” said Ziad Awad, from The Eye of the City, a publication in Syria’s Deir ez-Zor, which is cataloguing the IS massacres.