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Two mass graves containing at least 20 bodies have been discovered in the sewage system of this former Islamic State stronghold, a sickening reminder of the lasting devastation caused by the group’s three-year occupation of northern Iraq.

Human bones in the uniform of an Iraqi soldier and the remains of seven civilians were found Sunday at a site in the residential Ayadia neighborhood, under a home that once served as the local headquarters of the Islamic State (ISIS). Local officials who had been tipped to the presence of the remains on Friday said the victims were likely held in the basement of the home, which has yet to be "de-mined" and secured.

Across the street, at a second location, another 12 decomposed civilian bodies have been exhumed in recent days. Among the remains found were those of at least two children, and dismembered heads without other body parts.

Thick black cloths, likely used as blindfolds, and handcuffs were also found among the remains. Most of the victims appear to have been killed execution-style, with a bullet to the back of the head.

“It was a massacre. More and more of the same,” one police official told Fox News, which accompanied the recovery workers through their day's work on Sunday. “It is never-ending.”

Officials here have become all too familiar with the discovery of ISIS victims.

“Our duty is to the innocent people under the rubble,” Saad Hamadi al-Hussein, commander of the Nineveh Civil Defense Corps “Emergency Convoy” unit, told Fox News in an interview the night before the remains were exhumed. “ISIS kills them and throws them into the sewage one by one, and covers it over with cement. This is their way.”

“Our duty is to the innocent people under the rubble.”

— Saad Hamadi al-Hussein, commander of the Nineveh Civil Defense Corps “Emergency Convoy” unit

A team of roughly a dozen men in bright red suits from the convoy unit undertook the painstaking work, having been notified of the grave location less than two days earlier. At the first house, that began by cutting a four-foot square above a portion of the sewer. The sewer was then drained.

One person entered in a hazardous-materials suit and oxygen mask to survey the situation. The recovery then commenced with a few hooks, a ladder and gloved hands.

After several grueling hours, and with small children watching from low-lying rooftops above, 20 body bags were sealed and handed over to the Iraqi Federal Police. Law enforcement took them to a specialized committee established for examining the bodies. Whenever possible, surviving family members of the victims are notified.

Tel Afar, about 40 miles west of Mosul, had a population of about 100,000 before being captured by ISIS in June of 2014. The city was liberated last year, in October.

Remnants of war are nearly everywhere. Bodies found around and under homes are not uncommon. Neither is the smell of putrefied flesh, to those workers who spend their days and nights in the recovery efforts.

Hussein said that in many cases, families living under ISIS rule were forced to bury bodies in their own homes and backyards, as well as in local public squares. Civil defense officials have since begun recovering remains that can be identified for proper burial.

The 43-person Emergency Division is tasked only with removing civilian remains. A larger group, the 700-person-strong Nineveh Civil Defense Corps, sweeps up bodies from the streets, and collecting them from within individual homes.

The corps was established in 2003 under the U.S.-led Coalition Provincial Authority (CPA), and many received training from Americans in Bahrain. Members now say they don’t have adequate tools to do their jobs as effectively as possible, and claim requests for help, direct to both the Baghdad Central Government and international humanitarian organizations, have gone unanswered.

But the corps' situation is certainly better than it was under ISIS, when the group remained operational but its efforts were routinely sabotaged.

“Sometimes ISIS would make us go away after an attack, and they knew it was only civilians killed,” Hussein recalled. “Or if they knew their fighters were there, they would let us go in. Other times, they would come and force us to recover their dead fighters for them.”

For Daoud Salem Mahmood Ali, a 44-year-old rescue worker from Mosul -- he's considered something of a “hero” among his peers, since he’s been unearthing bodies for some 27 years – it’s a job that never gets easier.

“Sometimes, it’s five or six entire families all buried in one house,” he said after hosing himself off behind the fire truck at the end of a  shift in Tel Afar. “As a father, especially when I see women and children, it hurts. I have had to pull out many pregnant women, too.”

Ali said he and the team have recovered 2,400 bodies since Mosul was liberated last July, with more than 2,000 of those coming from the Old City, on the west side. In Tel Afar, they have since recovered about 640 bodies, many of whom are still unidentified. Another 500 bodies were discovered last year in a mass grave between the two strongholds, at the Badush prison.

But it isn't just recovery that's required of the dedicated team. Ali is also encumbered with “de-mining” bodies, typically those of ISIS fighters.

“I can tell straight away if they are ISIS. They are usually booby-trapped and often have foreign passports strapped to them,” Ali said. “We don’t remove them. Our command after de-mining is then to leave them, and authorities take those.”

But with an Iraqi federal election looming on Saturday, the cleanup and recovery effort in this blasted city is temporarily taking a back seat to politics. A large poster of the head of an Iran-backed group, striking a battlefield pose with the leader of the Iranian Revolutionary Guard’s Quds Force leader, adorns the gates at the city entrance. Other candidate posters are everywhere, even on bombed-out buildings.

The national focus on the campaign, Ali said, makes his work a mere afterthought to many.

“If it is our intention to put a poster in the street, cleaning will take at least another two years,” Ali said. “But if our intention is a humanitarian one, we can remove all the remains in a year.”