In the shadow of the international military campaign against the Islamic State (IS), Shia militias are operating with impunity as they kidnap and murder hundreds of mostly-Sunni victims across the crumbling expanse of Iraq, according to researchers at Amnesty International.

In a report released Monday, investigators at the rights group compiled a litany of abuses in the Iraqi cities of Baghdad, Samarra, and Kirkuk that they attribute to Shia militias. The crimes range from mafia-like abductions to atrocities carried out in revenge for IS attacks.

The militias, who count tens of thousands of members among their ranks and are organized in military units, often operate with the tacit or overt consent of the government in Baghdad. Previously a scourge of American occupying forces and, at times, the regime of former Prime Minister Nuri al-Maliki, militias have been enlisted to take part in the government's shaky fight against IS. In many cases, they are seen wearing official uniforms belonging to Iraqi security forces and riding alongside them in patrols.

The group most familiar to Americans is likely the Baghdad-based Mahdi army of Shia cleric Muqtada al-Sadr. Officially disbanded in 2008, Sadr's forces are once again active under the name Saraya al-Salam, or the "Peace Brigade."

Other groups, like the Badr Brigades, trace their roots to the 1980s, when they were first backed by the government of Shia-dominated Iran. The Asa'ib Ahl al-Haq (League of the Righteous), an offshoot of the Mahdi Army, is considered among the most powerful of the Shia militias, and reportedly maintains ties to members of Iran's Revolutionary Guard.

A spokesperson for Asa'ib Ahl al-Haq told reporters in June the militia's troops "are fighting side by side with the government's forces on all fronts," and openly admitted that they wore military uniforms, calling it "logical."

"There is a lot of close collaboration, these Shia militias are [sometimes] operating as formal Iraqi forces, wearing uniforms and driving military vehicles," Sunjeev Bery, Advocacy Director for Middle East and North Africa at Amnesty International, told VICE News. "It's difficult to know how much of the Iraqi central government's limited victories against ISIS are the result of the Shia militias, but they are a core part of the central government's strategy. That's what's most disturbing."

The case of Salem, a Baghdad businessman with nine children, illustrates the cynical approach of the militias. On the afternoon of July 15, Salem was reportedly kidnapped from a factory he owned about 20 miles from Baghdad. Salem's abductors contacted his family, and, after asking for $200,000, they eventually agreed on a $60,000 ransom in exchange for his freedom. Two weeks passed before his body was discovered among the corpses at Baghdad's morgue. "His head had been smashed and his hands were bound together with metal handcuffs," investigators wrote.

Salem's family told Amnesty that he had been arrested two weeks earlier, along with his son and brother, by members of the police and army who appeared in military vehicles, accompanied by a Shia man wearing a mask. The man told them to pay $30,000, which they bargained down to $27,000. That time, Salem was released.

It's unclear if Salem's kidnappers in each instance were linked, or if the second group of abductors even knew of his capture weeks earlier.

"Once you let loose the militias again, how do you stop them?" Judith S. Yaphe, Middle East Project Director in the Institute for National Strategic Studies at the National Defense University, told VICE News. "They are going to have an impact on the ability of the new government to negotiate, assuming it wants to, with Sunni leaders who may be wavering."

The families of other victims told Amnesty similar stories of militiamen demanding ransom payments that — even when paid — did nothing to forestall the death of their loved ones.

"Some militiamen are thieves as well as killers and try to get money from their victims' families, before killing them," a government official told Amnesty. "Those who are kidnapped by these have little chance of survival, no matter how much their families pay."

Though Sunni civilians have borne the brunt of the resurgent militias, abductions of Shia, Christians, and Kurdish civilians have also been documented. One Christian family said after being repeatedly targeted by militia members, they had no choice but to leave Iraq.

Kidnappers prefer Sunnis, however, if for no other reason than they can posthumously be labelled terrorists or IS-sympathizers without rousing suspicion.

Bery says families often refuse to approach authorities after kidnappings, worried that local police are in cahoots with the militias.

"They fear the local police will simply hand over the information the groups that carried out the abduction or crime in the first place," he said.

In Samarra, roughly 65 miles northwest of Baghdad, and where the Iraqi army in June repelled IS forces, Amnesty documented death squads that operated under the guise of security operations. Investigators counted at least 170 abductions of predominantly Sunni men since June. Amnesty researchers wrote that, on June 6 alone, "more than 30 were abducted from or near their homes, shot dead, and their bodies dumped nearby."

One women told investigators that armed men driving three Hummers broke into her family's home early on the morning of June 6 and drove off with their 22-year-old son, along with a neighbor's son. When the mother tried to follow, the men shot at her. "We looked for them everywhere until the following day, when their bodies were found in a mosque nearby," she said. "My son had been shot twice in the head and once in the chest."

"They didn't even know my son's name; maybe they just took him because they were looking for young men and he was the only young man in our house," the woman told Amnesty.

In Basra, UN officials have documented a similar pattern of abductions and killings, where in a two-month period between June 23 and August 20, 19 Sunni men — all civilians — were killed, along with 19 who were injured in attacks. UN officials reported threats made against Sunni mosques, and bulbs illuminating the entryways to Sunni houses have reportedly been painted black, while other homes have been marked with the letter X.

In July, gunmen with suspected links to Asa'ib Ahl al-Haq murdered 29 women in a Baghdad building thought to be operating as a brothel.

"I'm afraid we are regressing back to the situation as it was seven or eight years ago, when this behavior was very widespread," another government official told Amnesty.