A grave containing the bodies of at least 300 members of the security forces was discovered last November on the outskirts of Hamman al Alil, a town about 30 kilometres from Mosul.
But the scale of the killing at Khasfa dwarfs all other known sites. "Khasfa is definitely one of the biggest, if not the biggest, mass grave by Isil in Iraq.
And from what we have heard it is not just a grave but also an execution site," said Belkis Wille, senior Iraq researcher for Human Rights Watch. The organisation estimates that around 4,000 bodies are buried at the site.
Locals said the sinkhole was more than 400 metres deep before the dead began piling up.
The body count at Khasfa outstrips the most infamous massacre committed by Isil – the murder of army cadets at the Camp Speicher military base near Tikrit.
Isil militants butchered as many as 1,700 Shia recruits in the June 2014 massacre, turning the water of the Tigris red with blood.
At Khasfa, 2,000 policemen and soldiers were murdered in one day alone, claimed Mahmoud, who said he was forced to watch mass executions by the jihadists on four separate occasions.
"In the beginning, you couldn't see the corpses at the bottom of the hole. Only later, when it began to fill up could you see the bodies," he said.
The victims of one of the executions he was forced to watch included his cousin, who worked as a police office, he said.