— -- As the Iraqi military digs in against ISIS in the battle for Fallujah, American human rights advocates and Iraqi activists are voicing alarm about the potential for more sectarian atrocities from some Iraqi forces and their militia allies in the event of victory – a possible continuation of captives being tortured and executed with impunity, often on camera.
Over the weekend Iraqi officials confirmed that Iraq's controversial Popular Mobilization Forces [PMFs], Shiite-dominated militia groups, are participating in the fight for Fallujah, just west of Baghdad in central Iraq.
Human Rights Watch and the U.S. Departments of State and Defense have reported continued instances of war crimes over the past year in Sunni areas north of Baghdad such as Tikrit — showing that some groups in the military and militias on Baghdad's payroll have not stopped committing abuses since an ABC News investigation revealed widespread atrocities posted on social media 14 months ago.
Ali Khedery, a former senior U.S. official who was the longest-serving diplomat in Baghdad, said last week, "It will not surprise me if the city [Fallujah] is leveled and a lot of people are killed at the hands of the Iraqi security forces and militias."
If Shiite militias sack Fallujah, it will inspire revenge by Sunnis against Shiites in "a race to the bottom," he added.
Officially, the Pentagon says U.S. forces in Iraq do not support sectarian militias with airstrikes or otherwise assist them as Iraqi troops close in on Sunni-dominated cities and do not even know what specific role these irregular fighters will play in the Fallujah campaign.
But Pentagon spokesman and Navy Capt. Jeff Davis said last week, before the beginning of the battle for Fallujah, that PMF fighters positioned north of the city "have largely a relationship of co-existence with Iraq forces and are aligned against ISIS."
When it comes to the Iraqi military, another Pentagon spokesman, Marine Maj. Adriane Rankine Galloway, told ABC News last week that some Iraqi units and individual commanders continue to be barred from receiving U.S. military aid under a federal law that prohibits it from going to any foreign forces for which there is "credible evidence" of human rights violations.
Galloway said, "Some Iraqi units have been restricted from receiving assistance because their commander didn’t pass vetting" — echoing what officials told ABC News more than a year ago for an investigation that started in the dark corners of Iraqi social media.
[In an image posted on Instagram, six uniformed men, who appear to be Iraqi special operations forces from the Golden Brigades, surround an alleged ISIS suspect, who has been dragged with a rope or cable tied to his foot.]
Horrific Abuse, Executions Posted Online
The ABC News investigation broadcast in 2015, "Dirty Brigades: No Clean Hands in Iraq's ISIS Fight," included dozens of shocking photos and videos of torture, beheadings and roadside executions by Iraqi government special operation forces, Iraqi counterterrorism forces and special weapons and tactics police units trained and armed in many cases by the U.S.
Shiite militia allies were also often photographed carrying U.S.-made Colt M4 rifles manufactured in Connecticut or driving Humvees made in Indiana while torturing victims or proudly displaying severed heads. The U.S. equipment ended up in their hands presumably courtesy of the Iraqi military. In some U.S. military and Iraqi circles, the Iraqi units and militias in question are referred to as the "dirty brigades."
Shiite-dominated Iran is widely believed to support many of the PMFs. Philip Smyth, a University of Maryland researcher and an expert on Shiite militias and Iran, said he and many U.S. officials suspect that Iran’s notorious elite Quds Force in the Islamic Revolutionary Guards Corps was behind some of the accounts in the grotesque social media campaign — a twisted effort to build support for the militias after the Iraqi army suffered humiliation at the hands of ISIS in Mosul two years ago.
The barring of military aid to foreign allies on the basis of human rights abuses, unless the foreign government is bringing the responsible individuals to justice, falls under a vetting program operated by the Departments of Defense and State. ABC News last year disclosed that "certain Iraqi units," which U.S. officials declined to name for classification reasons, had been denied weapons and training under the Leahy Law because of suspected or known human rights violations.
Sen. Patrick Leahy, D-Vt., the law's author, said Iraq's government "should insist that members of the Iraqi security forces who commit such crimes are appropriately punished."
"Iraqi security forces have a history of violating human rights with impunity, and it is incumbent on the Departments of Defense and State to make every effort to prevent U.S. weapons from ending up in the wrong hands," he told ABC News last week.
[The patches worn by the men in one of the photos posted to Instagram appear to match the patches of the Iraqi Emergency Response Brigade, a counterterrorism unit under the Interior Ministry.]
Leahy isn't the lone voice noting the total lack of accountability in Baghdad for atrocities similar to those committed by ISIS — which the world has condemned.
The State Department's Bureau of Democracy, Human Rights and Labor last month strongly criticized the government of Prime Minister Haider al-Abadi, whose forces and militia allies committed atrocities and got away with it, according to an annual human rights assessment.
"Numerous reports continued during the year  of Shia PMF killing, torturing, kidnapping and extorting civilians," the State Department country report on Iraq said. "Security forces reportedly committed extrajudicial killings, although identification of specific killers was rare."
The Iraqi government "rarely investigated" allegations of human rights violations in 2015, the State Department report said, even when presented with photographic evidence and eyewitnesses by organizations such as Human Rights Watch and journalists at ABC News.
[A uniformed individual holds the severed head of a purported Saudi ISIS fighter atop a U.S.-made Humvee in Iraq. In the background, a man wears two patches signifying the Iraqi Interior Ministry’s Emergency Response Brigade.]
'Do You Want a Civil War in the Streets?'
Last year both the Iraqi military and al-Abadi promised to investigate and hold accountable anyone committing war crimes, though the vast majority have been perpetrated by ISIS rather than the government. Repeated inquiries over the past 14 months, including last week, by ABC News to Baghdad Operations Center spokesman Gen. Saad Maan about any results have gone unanswered.
"Information about investigations or prosecutions of abuses by government officials and members of the security forces was not publicly available. Impunity effectively existed for government officials and security forces personnel," the State Department report said.
Joe Stork, a senior investigator at Human Rights Watch, recently visited Baghdad and went straight to the top to address the fact that no one has been held responsible for war crimes.
"I met with the Prime Minister Abadi and raised the impunity and accountability issues," Stork told ABC News. "He said he shared our concerns. We asked why there has been no accountability, and he said, 'Do you want a civil war in the streets?'"
A social media superstar in the Shiite militias is the gregarious weightlifter known as Abu Azrael (the Angel of Death), a fighter wielding a U.S.-made M4 who has declared he will "grind to dust" ISIS jihadis in Fallujah. He appeared in a new video last week with militia fighters, comparing the coming Fallujah offensive to the successful liberation of Baiji he participated in last year.
But it was in Baiji where Abu Azrael — called the Shiite Rambo by his fans — made international headlines last August, when he used a sword to slice off pieces of a charred alleged ISIS fighter strung up by his feet by PMF troops.
Asked about the incident, Basam al-Hussaini, the representative in the U.S. of the PMFs, said Iraq "is not a civilized, perfect country. We can only do so much."
Weeks after the Abu Azrael incident, the top U.S. envoy in the region, Brett McGurk, praised the militias under the government's umbrella for the fight to retake Baiji in northern Iraq. "The U.S. commends progress by Iraqi security forces & Popular Mobilization Forces against #ISIL terrorists in #Bayji," he said on Twitter.
Sunni activists, therefore, are especially alarmed amid Iraq’s ongoing offensive against ISIS. They fear Shiite retribution against their people in western Iraq after ISIS fighters are driven out of Sunni cities and towns, where they have received varying degrees of food and succor.
"We firmly oppose any involvement of Shiite militias to liberate Fallujah, whether through their participation in operations or under the umbrella of the Iraqi security forces, as was the case in Ramadi last year," a prominent Sunni activist, Sheikh Khamis al-Khanjar from the Office of the Arab-Sunni Representative for Iraq, said in a statement last week.
Al-Khanjar accused top leaders from two prominent Iranian-backed militias — the Badr Organization and Kata'ib Hezbollah, known for killing many U.S. troops in the Iraq War of 2003 to 2011 — of being "involved in crimes against humanity" in past operations.
A Human Rights Watch report published last September said that in the case of Tikrit, after it was retaken by Iraqi forces, "officials and residents in Tikrit also alleged that the militias were involved in widespread looting and extrajudicial killings."
Al-Hussaini, a Shiite Iraqi-American, told ABC News that militias "are involved in Fallujah" but only a "tiny" percentage of them participated in war crimes.
"There are going to be some bad apples, whether you are an American soldier or PMU [Popular Mobilization Unit]. It's going to happen," he said in an interview. "There is no control. It's a battlefield. It is chaos."
ABC News' Luis Martinez and Lee Ferran contributed to this report.