For months, ISIS fighters drove thousands of civilians on forced marches across the Nineveh desert into the small town of Hamam al-Alil. Retreating ahead of methodical Iraqi advances on Mosul's southern approach, ISIS fighters converged here, rounding up men, women and children for use as human shields and killing dozens of others.

When Iraqi forces began to close in on this cluster of villages along the Tigris River valley, most of the militants fled into Mosul, taking thousands of civilians with them. But before the retreat, ISIS fighters also led hundreds to a garbage dump past an old ISIS training camp and shot them dead, leaving the bodies among the piles of trash.

A week after Hamam al-Alil was retaken from ISIS, and days after a delegation from the central government in Baghdad visited the site, about a dozen bodies remain strewn among piles of garbage on the western edge of the town. The bodies that remained were the ones family members were unable to identify. Some had been decapitated, other have their hands and feet bound. Iraqi officials at the scene said the men were killed for alleged spying in aid of the operation to retake Mosul or having links to the Iraqi government's security forces.

No efforts to preserve the site were visible on a visit Friday and Iraqi officers reported that wild dogs were eating at the decaying corpses that lay on the edge of an old agriculture college later bombed by coalition aircraft after ISIS converted the sprawling compound into a training base.

"These were men working with us," said Iraqi federal police Cpt. Muhannad Adnan.

"This is the clearest evidence of the crimes and brutality of Daesh," he continued, "it's also evidence of what happens when we slow our advance. When we move slowly, it allows (ISIS) to plan better and kill more people."

Adnan said the town's residents have told his unit that dozens more bodies are buried at the site.

Iraqi forces south of Mosul has taken a month to advance some 25 miles, slowly pushing ISIS fighters out of dozens of small villages along the Tigris River and across the desert. On Mosul's east, forces initially advanced faster, but have slowed recently as they have pushed into the city proper.

Many Iraqi forces have largely paused their advance on Mosul over the past four days. Iraqi commanders say they are using the time to consolidate territorial gains ahead of future advances.

"Our duty should only be to liberate territory and then continue to move forward," said Brig. Gen. Shaker Alwan al-Kafaj, the head of the federal police's fifth division.

"The local police forces had more than a year to prepare for this and now we are finding out that they did almost nothing. Many of them don't even exist," he added referring to the problem of so-called ghost soldiers — non-existent troops whose pay is pocketed by senior officers or enlisted men who don't show up to duty — that has plagued Iraq's security services for years.

Al-Kafaj has been leading advances south of Mosul since the operation was officially announced on Oct. 17, and says his momentum forward has repeatedly been delayed by inadequate forces to hold the newly gained territory.

Most of the dead found on the western edge of the town were former ISIS prisoners, according to federal police commanders. The building used by the extremists as a prison stood in central Hamam al-Alil on a street of brightly painted homes with fruit trees in their front gardens. Inside, walls were painted black, windows were bricked up and five crude solitary cells were built in a stair well.

"They didn't want prisoners to know if it was day or night," said Imad Ali, a resident who lived down the street from the prison.

Many of the people kept here were men and women charged with relatively minor offenses like smoking cigarettes or breaking the militant group's strict Islamic dress code, according to the federal police officers and residents. People charged with more serious crimes like spying were killed, Ali said.

A front room next to the kitchen was where the interrogations would take place. Behind that was the room where female prisoners were kept. Before ISIS fled, the fighters set the building on fire.

"They wanted to burn the evidence of their crimes," Ali said.

During the last days of ISIS rule in Hamam al-Alil, Iraqi federal police Cpt. Oday al-Jabouri's wife, children and brother were arrested by ISIS fighters.

Al-Jabouri's family has strong ties to Iraq's security services and his brother had previously been a local police officer before ISIS took over the town. "They arrested him before they fled because they didn't want him to help (Iraqi government forces)," al-Jabouri said.

He said he heard from a friend inside Mosul that his wife and children were taken into Mosul by ISIS fighters to be used as human shields, but no one has seen his brother since he was arrested.

"We didn't find his body at the mass grave, so he could still be alive," al-Jabouri said, "but we haven't looked beneath the ground yet."