 -- U.S.-trained and armed Iraqi military units, the key to the American strategy against ISIS, are under investigation for committing some of the same atrocities as the terror group, American and Iraqi officials told ABC News. Some Iraqi units have already been cut off from U.S. assistance over "credible" human rights violations, according to a senior military official on the Pentagon's Joint Staff.

The investigation, being conducted by the Iraqi government, was launched after officials were confronted with numerous allegations of “war crimes,” based in part on dozens of ghastly videos and still photos that appear to show uniformed soldiers from some of Iraq's most elite units and militia members massacring civilians, torturing and executing prisoners, and displaying severed heads.

The videos and photos are part of a trove of disturbing images that ABC News discovered has been circulating within the dark corners of Iraqi social media since last summer. In some U.S. military and Iraqi circles, the Iraqi units and militias under scrutiny are referred to as the "dirty brigades."

“As the ISF [Iraqi Security Forces] and militias reclaim territory, their behavior must be above reproach or they risk being painted with the same brush as ISIL [ISIS] fighters,” said a statement to ABC News from the U.S. government. “If these allegations are confirmed, those found responsible must be held accountable."

[In an image posted on Instagram, six black-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.]

Sen. Patrick Leahy, D-Vermont, along with international human rights advocates and military experts, called the photos evidence of Iraqi "war crimes."

"I guarantee you ultimately we get blamed for it whether we did it or not," Leahy predicted.

Under what is known as the Leahy Law, the U.S. is required to cut off funds to any foreign military unit when there is “credible evidence” of human rights violations. In Iraq the responsibility of determination falls to the Department of Defense. In recent Senate testimony, Secretary of State John Kerry confirmed the Iraqi investigation had been ordered and said the Leahy Law applies to units operating alongside the many militias also fighting in Iraq against ISIS.

"I would say that involves the Leahy Law," Leahy recently told ABC News after viewing the shocking imagery. “And I'd argue that we should be withholding money."

According to the Pentagon, the U.S. already has. In a statement to ABC News, the Joint Staff official revealed that in the months since the U.S. began airstrikes and military assistance to Iraq last August, “We have withheld assistance from certain Iraqi units on the basis of credible information in the past. Due to the sensitive nature of our security assistance, we are unable to discuss specific units.”

In Washington today, Chairman of the Joint Chiefs Gen. Martin Dempsey told lawmakers the U.S. military is keeping a close eye on the militias as well.

"What we are watching carefully is whether the militias -- they call themselves the popular mobilization forces -- whether when they recapture lost territory, whether they engage in acts of retribution and ethnic cleansing," he said.

An Iraqi government spokesperson previously said while the dozens of photos could be ISIS propaganda, a full investigation was warranted.

“Yes, of course we will investigate these pictures,” the spokesperson, Gen. Saad Maan, said in an interview in Baghdad as he viewed a selection of images provided by ABC News.

"We don't have anything to hide,” the general said. “We don't have anything to be in, let's say, in a black corner."



[A bound and blindfolded detainee appears to be dropped – or possibly hung from the neck according to one analyst -- from what looks like an Iraqi military base guard tower. The image was posted on Instagram.]

The Iraqi military is key to the U.S. strategy to fight ISIS and stop its atrocities, which have outraged the world. The U.S. is shipping almost $1 billion in weapons, as well as providing U.S. military trainers to instruct new Iraqi recruits. A special operations official in Baghdad, however, said it’s the government of Iraq that decides — not the Pentagon — which Iraqi units get U.S.-donated weapons, such as 43,000 M4 rifles and thousands of other light infantry weapons Congress approved for shipment in December. American troops are not known to be operating on the ground in combat in Iraq or Syria. No Americans are shown in the images or footage ABC News has found, nor have any Americans been implicated in any of the alleged atrocities.

Officials from Human Rights Watch and Amnesty International who reviewed the library of horrors assembled in the ABC News investigation said it is rare to see so much visual evidence of human rights abuses.

"Usually when forces commit such crimes they try to hide them. What we are seeing here is a brazen, proud display of these terrible crimes," Sarah Leah Whitson, Middle East Executive Director at Human Rights Watch, said in an interview as she and the group's lead investigator in Iraq, Erin Evers, surveyed the carnage.

ABC News came upon the first such images last September, when a reporter following personal Instagram accounts of Iraqi counter-terrorism troops spotted a video of a handcuffed prisoner shot in the head by a man in camouflage -- which more than 600 users "liked." The English and Arabic captions by a self-identified member of the Iraqi security forces said, “We have arrested this terrorist yesterday and we killed him after completion of interrogation."

A separate photo posted in September showed the severed head of a long-haired and bearded alleged ISIS fighter lashed to the grill of a U.S.-donated Humvee bearing an Iraqi Army license plate. A second related photo eventually surfaced of what appeared to be an Iraqi Army soldier holding up the same severed head next to the gun truck. Desecration of war dead and extrajudicial killings are violations of the Geneva Conventions.

"You don’t behead someone and place their head on the front of your Humvee. That’s unacceptable -- because it’s a war crime. And it’s an atrocity," retired U.S. Army Special Forces Lt. Col. James Gavrilis told ABC News.

As a senior officer in 5th Special Forces Group in Iraq a decade ago, Gavrilis was deeply involved in counterinsurgency during the U.S. war and creating Iraqi counter-terrorism units from Special Forces and special police teams.

"I think it’s horrible. I think this really shows a failure of our policy for Iraq," Gavrilis said, confirming that the imagery looked authentic and too plentiful online to be faked.

"Both sides are committing war crimes," he said. "This is widespread, it’s endemic."



[A man wearing a uniform with a patch that appears to be from the Emergency Response Brigade steps on two severed heads in a photo posted on Instagram.]

In another video posted online in October, two unarmed civilians are shot to death after being questioned, and denying, whether they were part of ISIS. When the camera pans to one man with a gun, he appears to be wearing a uniform and shoulder patch of Iraqi Special Forces, with Iraqi Army officers also nearby observing the atrocity.

Fighters who appear to be a mix of militia and army appearing in a separate 78-second video circulating in January — including some wearing Iraqi flags and Iraqi Special Forces patches — take pictures of a captured teenaged boy who appears terrified. “Didn’t you just shoot?” demands one fighter. The handcuffed boy, shoved to the ground, insists, “No, no, I did not shoot a single bullet.”

The men argue over whether to kill him, some asking the others to calm down, but they shoot him to death anyway as the sound of mortars and gunfire nearby punctuate the crime. “This is to avenge the martyrs,” one man says.

“I've seen all sorts of horrible things over the years... but I have never seen anything this bad in my life,” said Ali Khedery, an American former diplomat in Baghdad who advised five U.S. ambassadors in the Iraqi capital and three generals overseeing Middle East operations at U.S. Central Command.

Khedery recently wrote in Foreign Policy about another video, where a man was beaten and machine gunned to death by a gang who appeared to be both militias and Iraqi Special Forces with U.S.-donated M4A1 rifles. He said the video slaughter of the Iraqis accused by their killers of smuggling weapons for ISIS was far worse, because Iraqi government troops were present.

“It was the shooting of unarmed men. This is a U.S.-backed government. They carried U.S. weapons,” he said.

U.S. and Iraqi authorities say they have been working to fully authenticate the content posted online on sites such as Facebook, Instagram and Twitter connected to the Iraqi military. The uniforms and insignia of Iraqi Special Operations Forces under the command of Baghdad’s Counter-Terrorism Forces as well as special police and Emergency Response units from the Ministry of the Interior are clearly identifiable in many of the photos and videos, which include many severed heads and corpses dragged behind humvees.

Gen. Maan, the Iraqi government spokesperson, claimed the patches identifying Iraqi military units could be bought on Iraqi streets and that the gruesome images could be a clever ploy by ISIS to discredit the Iraqi military.



[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 (E.R.B.), a counter-terrorism unit within the Ministry of Interior.]

"It does not look like ISIS propaganda at all," Gavrilis said. "I don’t know how we could support them, if they are spearheading a lot on the front lines alongside these militias, and if they are conducting these kinds of atrocities as well... These Shi’a militias are just as barbaric as ISIS."

Some militias take pride in their atrocities and appear to often be calling the shots on the battlefield, not the government forces, BloombergView columnist Eli Lake found when he recently visited the front lines north of Baghdad.

Officials said that the State Department's human rights observers and military intelligence had viewed examples of Iraqi Security Forces posting atrocities on personal social media for over a year. But one knowledgeable U.S. official said that since ABC News began asking about the many disturbing images last fall, the atrocities allegations against Iraq’s fighting forces have grown “more severe” and the “very concerning” allegations are being raised at high levels in Baghdad.

The Pentagon spokesperson told ABC News the U.S. military has "discussed with Iraqi leaders the paramount importance of maintaining high standards of conduct and protecting civilian populations of all sects."

"The actions of a small minority, if left unchecked, could do serious harm to the efforts of the Iraqi government," the spokesperson said.

With several thousand American troops back in Iraq as trainers, the alleged atrocities by Iraqi troops puts U.S. military commanders in the unenviable position of having to sort out which units are clean or dirty, Gavrilis said.

[The severed head of an alleged ISIS fighter is being held up by a desert camouflage-uniformed individual in front of a Humvee in this image uploaded to Instagram. Patches on his uniform match those often worn by the Iraqi Army.]

The UN High Commissioner for Human Rights last month released a report on Iraq that both condemned ISIS for its campaign of killings verging on genocide, but also criticized Iraqi Security Forces for military operations that "which may have amounted to war crimes."

Last March, the State Department's Bureau of Democracy, Human Rights and Labor also issued its own damning report on Iraq, stating that government officials under then-Prime Minister Nouri al-Maliki committed "extrajudicial killings" -- meaning battlefield executions of ISIS suspects and killing individuals in custody without trial.

"Ministry of Interior officials tortured detainees to death, according to reports from multiple government officials and human rights organizations," read the annual report. The Bureau explicitly fingered the Iraqi Counter-Terrorism Forces and Interior Ministry's special police units -- which the U.S. established, trained and armed from 2003-2011, and whose troops are seen in many of the atrocities images.

But the State report was issued before the U.S. began airstrikes in Iraq last August to assist security forces in successfully retaking the Mosul Dam, and long before President Obama deployed thousands of American infantrymen, special operations forces and enablers back into Iraq beginning last fall to assist the Iraqis in fighting ISIS. A new report is expected soon, officials said.

Now that the alleged war crimes of the U.S.-backed forces have become public, the Iraqi spokesman stressed that his government will not tolerate “bad behavior.”

Using the Arabic slang for ISIS, Gen. Maan said, "We do not allow any person to be a savage like Daesh."

ABC News' Divya Kumar, Cho Park, Rhonda Schwartz, Randy Kreider and Lee Ferran contributed to this report.

[A black uniformed individual holds the severed head of a purported Saudi ISIS fighter while standing on top of a black-painted Humvee. In the background, a man wears two patches signifying the Iraqi Interior Ministry’s Emergency Response Brigade.]