It was earlier this year when I met Dr Mansour Maroof Mansour.

Standing in the morgue of the General Hospital in Qayyarah, an oil town that sits just south of Mosul, the doctor described the daily struggle he and his staff faced, coping with the numbers of dead and wounded ferried out from the battles then raging in the west of Iraq’s second largest city.

“On one occasion we had hundreds of people, not just those suffering gun and blast injuries, but, many who had inhaled toxic gas from the sulphur plant that Daesh, set on fire near here,” he recalled, using the Arabic term for Islamic State (IS).

Walking through the multi-storied building of the hospital with its dilapidated corridors and wards, Mansour told me of the conditions his staff confronted when they reoccupied the hospital, after it was retaken from IS by the Iraqi Army.

“Surgical instruments, equipment, anything they could transport was moved, probably back to Mosul,” he explained, adding that retreating IS fighters had also rigged the hospital with booby traps and bombs before their departure.

Today the battle for Mosul is over, but life for Mansour and his staff is no less grim and demanding. Sitting as it does on the banks of the Tigris River downstream from Mosul, Qayyarah hospital and its morgue continues to overflow with bodies.

Most of the dead are now found floating in the river or washed up on its banks. Many have their hands bound, eyes blindfolded and have been killed by a bullet to the back of the head.

The ones that have been dead longest float on the surface, buoyed by the gasses produced as they decompose. Those killed more recently drift in the current that pulls them down the river.

There are contradictory versions of the story as to who these floating dead are - some say they are victims of IS, who killed hundreds if not thousands of opponents and civilians as the city was retaken by Iraqi security forces.

Others, however, say the evidence points elsewhere - toward extra-judicial killings of local Mosul residents, accused of having joined IS, now that the territory is under Iraqi government control.

“We can’t identify the bodies in the river. They are very decomposed and there is nothing on them to use for identification,” Mansour was recently quoted as saying in an interview, as he stood surrounded by body bags in the same morgue where I talked with him a few months ago.

If the identity of the dead remains something of a mystery, then so too does that of their killers. Or does it? In the few weeks since Iraqi forces declared victory over IS in Mosul, human rights groups have been compiling evidence that tells a grisly counter story to the scenes of celebration that showed joyous Iraqi soldiers and a visit to ‘liberated’ Mosul from Iraqi Prime Minister Haider al-Abadi.

Almost daily, accounts grow of extrajudicial killings, torture and unlawful detention by Iraqi forces in the final phase of the battle to retake the city and since.

In some instances the atrocities are no secret, with videos and photographs surfacing on social media of Iraqi soldiers participating in torture and revenge executions of IS suspects.

However, even before the Mosul battle was over, and this more damning evidence appeared, some officials were already providing an explanation for the bodies appearing in the Tigris

“Most of it is score-settling,” one intelligence officer in Qayyarah was quoted as saying back in April this year as the campaign to take the city reached its height. “That includes the killing of IS suspects by elements of the security forces.”

Should the security forces indeed prove responsible for some of the killings, then their actions in some instances must been seen in the context of what they themselves encountered during the bitter no-quarter struggle for the city.

Just last week one soldier approached what he thought was an IS corpse. The militant was pretending to be dead and shot the soldier at close range with a pistol. The online news portal Middle East Eye (MEE) also reported how four IS members, two foreign fighters and two Iraqis, were found hiding under the rubble. According to an Iraqi soldier stationed in the district who spoke to MEE reporters, all four IS fighters were shot.

The brutality of the long drawn out battle to retake Mosul and other Iraqi cities and towns from IS and the barbarity of the jihadists, have taken their toll on the Iraqi armed forces. In the ruins of Mosul there is little humanity left among many combatants.

The line too between extra-judicial killing by the state and private vigilantism is blurred because many members of the security forces have been personally affected by IS.

Some undoubtedly wish to avenge the deaths of comrades, friends, family and relatives, blown apart, gunned down or tortured by IS in Mosul’s streets.

For many Iraqis who suffered more than two years of extreme violence and privation under IS, no punishment is too severe for people who joined them.

But as human rights workers correctly point out, two wrongs do not make a right and revenge and reprisal killings only help fuel the instability that grips the country.

Only last week, international observers from Human Rights Watch (HRW) reported the discovery of an execution site in Mosul and gave warning of a resurgence of violence if no one is held to account over the murder and torture of Iraqis in the battle against IS.

The site was first discovered on July 17 when a shopkeeper brought the observers to an empty building containing 17 male corpses, barefoot but in civilian dress, surrounded by pools of blood, in the west of Mosul’s Old City.

International observers reported seeing soldiers from Iraq’s Counter Terrorism Service in west Mosul around the time of their visit to the execution site. The shopkeeper also reported seeing Iraqi security forces' 16th division in the area four nights before uncovering the corpses. The night before the discovery, he said, he heard gunshots coming from an area near the empty building.

“As Prime Minister Abadi enjoys victory in Mosul, he is ignoring the flood of evidence of his soldiers' committing vicious war crimes in the very city he’s promised to liberate,” said Sarah Leah Whitson, HRW's Middle East director in response to the discovery.

“Abadi’s victory will collapse unless he takes concrete steps to end the grotesque abuses by his own security forces.”

The report on the execution site from HRW comes just days after four videos allegedly showed Iraqi troops killing and beating detained militants. An Iraqi man who frequently reports on military activities in the Mosul area released the footage on social media. One video clip showed men in Iraqi Army uniforms throwing a detainee off a cliff onto the banks of a river and opening fire on him. As he fell he landed next to another motionless body.

HRW later verified the location of the first video using satellite imagery. A second video shows a uniformed man shooting an unarmed detainee, while the remaining two videos show soldiers beating and kicking detainees.

Eyewitnesses in Mosul’s old city where the most stubborn IS resistance took place and which saw some of the worst fighting, say new corpses still appear at different locations. Some have clearly been executed, shot in the head at close range.

Many still have ropes trailing from tied hands and feet, indicating that either while dead or alive, they were dragged through the deserted streets. Little continues to be done by the Iraqi authorities to investigate the killings however.

To date Prime Minister Abadi has insisted that any Iraqi soldiers who violate instructions to hand over prisoners will be tried in a military court, but still the bodies keep turning up in the Tigris and elsewhere.

Abadi has also referred to the killings as “individual acts” and called on human rights groups to “check and verify their sources.”

Another disturbing aspect to all of this is that US troops embedded with the Iraqi military seem to have seen almost none of these killings and atrocities. In fact fewer that five instances have been reported despite the presence of US troops with Iraqi security forces throughout the Mosul campaign.

As a BuzzFeed news report recently asked, does this mean that US forces have simply been absent when the most horrific abuses have taken place or are they ignoring them?

“I think they’re ignoring it,” one Kurdish security official watching the post-Mosul fallout from the nearby regional capital of Erbil replied to BuzzFeed.

“It’s no secret. The bodies are turning up in the Tigris River. They’re being shot on the roadside. Groups are going around at night and taking people out. This is what post-Mosul is. This is just the beginning.”

The same official made the point that US policy in Iraq now has different goals from when the US first occupied the country hoping to turn it into a model democracy.

“Remember the US is not here for nation building,” he said. “It’s not 2003. I think their singular objective is to take out IS. And if that means the Iraqis are going to take part in extrajudicial killings, I don’t think it’s their place to get involved. What can they do? I think they find themselves in a very difficult position.”

Some human rights watchers like Belkis Wille, a senior Iraq researcher for HRW, say that the US simply doesn’t want to know that some of the forces they are supporting might find themselves on a human rights blacklist.

“The horrors that the people of Mosul have witnessed and the disregard for human life by all parties to this conflict must not go unpunished,” insisted Lynn Maalouf, another rights researcher working for Amnesty International.

Few doubt too that many of the killings have been sectarian motivated. Ever since 2003 Iraqi forces and mostly Shia non-state and government armed groups have carried out abuses against the civilian population with complete impunity, mainly targeting Sunni Arabs.

Mosul is no exception and many of those killed may not have been IS members of even sympathisers, but simply civilians murdered because they were Sunnis.

Again, international observers point to the presence of Shia paramilitary groups or Popular Mobilisation Units (PMU), who fought alongside and in support of the Iraqi Army, as possibly having taken the law into their own hands, committing reprisal attacks on Sunnis in Mosul and elsewhere.

“PMU militias have carried out a systematic pattern of violations, including enforced disappearance, extrajudicial executions and other unlawful killings and torture of Sunni Arab men and boys, seemingly in revenge for IS attacks,” said Amnesty International in a report earlier this year.

Revenge, reprisal, vigilantism or sectarian motivated, whatever the reasons behind the killings taking place in the wake of the Mosul campaign they all bode ill for Iraq’s future.

Put quiet simply the future of Iraq is at stake. Left unchecked this surge of revenge killings could haunt Iraq for generations to come. Many Iraq analysts rightly flag up that IS exploited Iraq’s sectarian tensions to facilitate its rise to power in the first place.

Many Sunnis who felt disenfranchised by Iraq’s Shia majority government and threatened by Shia non-state armed groups welcomed or at least tolerated IS’s arrival in 2014.

Any continuation of what is underway right now in Mosul will only make it easy for the next version of IS to gain traction and recruit.

Back in Qayyarah, the town south of Mosul, the bodies continue to pile up in the morgue at the hospital run by Dr Mansour.

Not far from the hospital sits a house locals say once belonged to Ali Khether, a well-known IS commander who lived there when the jihadists occupied Qayyarah.

On one wall of the house the same locals have daubed graffiti that reads: “Blood for blood.” This is the prevailing mood around much of Mosul right now. It may yet prove to be the undoing of Iraq.