Transcript for Elite Iraqi troops torture, execute civilians in footage captured by photojournalist
We begin with an ABC news investigation in a place where cameras rarely go, inside the fight against ISIS. This story involves a photo journalist embedded with Iraqi soldiers working with America to take down ISIS. But what this reporter witnessed on the front lines was far from heroic. He saw these U.S. Allies torturing and murdering civilians, behavior that's being described as nothing short of sadistic. Here's ABC's chief investigative correspondent Brian Ross. Reporter: In the fight to destroy ISIS, these elite Iraqi soldiers have been appraispraised by the U.S. As helping to lead the charge. These are the men of what's called the Iraqi emergency response division. The erd. Their battlefield exploits captured in this combat video licensed by ABC news. They are against ISIS. Reporter: The hours of video and still photos are the result of great battlefield courage by this Iraqi photo journalist, Ali akadi, who was embedded last year with the emergency response division. But tonight akadi is showing another kind of courage, blowing the whistle on soldiers he followed and became friends with. Revealing for the first time graphic scenes of torture and murder of civilians. Some too horrific to be broadcast. It all raises the question of whether the U.S. Is turning a blind eye. Was this happening all the time? This is happening all the time. Reporter: Arkadi is an award-winning photo journalist whose work for the respected agency 7 shows the human side of war. The victims. The orphans. The soldiers. I'm photo journalist from Iraq -- Reporter: Invited to travel with the emergency response division, his original idea was to feature these two Iraqi soldiers as heroes. A captain from the Sunni sect of Islam. And a corporal from the shia sect. So your idea was to show a positive story. Yeah. People from both sides of the religion. Yeah. Working together against ISIS. Yeah. Reporter: Arkadi went everywhere. Night and day. With the erd. From Fallujah to mosul. As time went by they began to trust you? Yes. I like the people's personalities, very good guys. It was a very, very good group. Reporter: But it was on this night raid outside mosul last November when arkadi says he began to realize the soldiers he was following were no heroes. Entering the home of a family, they ignore a crying mother and her children. You're scaring the children, she says. They pull the husband outside and begin to beat him. Claiming he and his wife once helped ISIS. And then the soldiers, including captain OMAR Nazar, the Sunni officer who arkadi was going to focus on for his positive story, forced the captain to repeat a pledge of loyalty to the ISIS leader Abu Bakr Al baghdadi. According to arkadi, the captain wanted him to later edit the tape so it would appear the man knew the ISIS pledge without prompting in order to justify killing him. They knew you had this video? Yeah. They knew if it got out, there would be trouble? Yeah. Reporter: And it got worse. The very next day, arkadi and his camera were present when the unit methodically prepared to torture this man, a sheep herder whose teenage sons were suspected of working for ISIS. As a senior officer gives the directions, "When I tell you to kick, you let him go." The soldiers close the curtains but allow arkadi to continue filming the ugly scene. It's extremely disturbing. Reporter: We showed the footage to Sara Lee Woodson of human rights watch. It's terrible torture, horrible torture. This is actually happening under command authority. This is someone with some level of authority who has the jurisdiction to tell soldiers what to do, telling his soldiers how to torture this detainee. Reporter: Seemingly confident the photo journalist would never expose them. Not shy at all, in fact, they're smiling. And frankly, there's not even a pretext here of torture in the name of obtaining intelligence. This is just torture for fun. Sadistic? Sadistic. Reporter: In a statement to ABC news, the emergency response division said the video images are fabrications created by ISIS to make Iraqi soldiers look bad. But later in a remarkable phone call with ABC news, captain Nazar admitted the video was real and said he had nothing to apologize for. That he can tell in ten minutes or less, who is and who is not ISIS. Translator: We have made mistakes but they were all directed toward the enemy, ISIS, and I'm proud of those mistakes. It's no different than what ISIS does, on the same moral equivalency of what they do. Reporter: We showed the video to retired green beret lieutenant colonel Scott Mann, who served in Iraq and elsewhere as a counter againinsurgency adviser and helped train special operations officers. If you're asking if there's a place for that on the modern battlefield, absolutely not. This is the powder keg for strategic failure. This is the narrative fuel that groups like ISIS look for. This is really counterproductive in the fight against ISIS? It's totally counterproducti counterproductive. Reporter: So the new revealing footage we obtained raises questions of what U.S. Military officials know or tolerate. On this night, the torture by the erd unit of two Iraqi brothers was led by a soldier described to arkadi as the unit's liaison with the U.S. Military. The torturer himself told arkadi that American soldiers taught him this so-called interrogation technique, applying the point of a knife to a sensitive spot behind the ear. The two brothers, one a car salesman, the other who owned a falafel stand, were supposedly under suspicion as ISIS supporters. Although arkadi says the soldiers knew they actually had escaped from ISIS. They're not ISIS? They're not ISIS. Reporter: A few days later, arkadi received this cell phone video from an erd soldier showing the dead body of one of the brothers who was tortured. On the video, a soldier says "We took revenge." In our phone call, captain Nazar said his group has instructions not to take prisoners. Translator: We do not want prisoners from ISIS, I'm telling you, we don't take prisoners." What should the U.S. Reaction be the official Pentagon response to these images we sent to them? Well, it should be rep ra hengs, the same we feel right now. Based on what I've seen here, that can't be allowed to persist. And there should be punishment for anyone that's connected -- this is a violati of law and warfare. Reporter: In a statement the American embassy in Baghdad told ABC news the U.S. Has not provided military aid, arms or assistance to the emergency response division. Apparently because of previous human rights violations. Yet the footage we obtained shows erd soldiers with the very anti-tank weapons the U.S. Has been providing to Iraqi forces. And earlier this year, a senior U.S. Military official in Baghdad, colonel Brett Sylvia, publicly praised the erd in a briefing for reporters back at the Pentagon. They have proven to be a very effective fighting force. This is the first time that we have advised them, and it has been a really fruitful partnership in all regards. Reporter: The two soldiers who invited arkadi to follow them, captain Nazar and corporal haydr Ali, have developed a folk hero status in Iraq, the subject of a social media video called "Happy Baghdad" produced by arkady. At one point arkady became very close with the two men. To his shame, arkady admitted to us after our interview that he himself did go along twice and struck captives. But now arkady has fled Iraq, breaking out his graphic video, his family in hiding after receiving death threats from the two soldiers he once considered friends. Do you think they would kill you? Yeah, why not? It's easy. Would you ever be able to go back to Iraq? I don't know. Really, I don't know. Reporter: Of all the images ar arkady showed us, the most disturbing may be one the soldiers themselves recorded on a phone and sent to him. It shows two soldiers chasing down a handcuffed prisoner and then carrying out a brutal field execution. That's extra judicial killing. I don't even have words for it. That's a murder? That's a murder. Reporter: In our phone call, captain Nazar told us human rights don't apply to people that he suspects of being ISIS. . Translator: He's not human, he's a monster. Isn't that murder? Translator: No, it's not considered murder. Is there now going to be an investigation? Translator: I'm already a star. Ali would make me a bigger star by doing this. Why? My country is longing for someone who would help it get rid of terrorism. Reporter: The soldiers thought their secrets were safe with arkady. Tonight they are learning they are not. For "Nightline," Brian Ross, ABC news, New York. Our thanks to Brian Ross for that report.
This transcript has been automatically generated and may not be 100% accurate.