The headless body was lying in a pool of blood in the street, partially covered by a tan blanket.
An Iraqi military commander said the man was one of eight
Allegations of extrajudicial killings by Iraqi government forces surfaced recently, nearly a month after the offensive to recapture this northern city began.
A video shared widely on social media Friday appears to show Iraqi men in uniforms shooting and then driving a tank over a youth they accuse of fighting for Islamic State.
It was not clear where the video was recorded, and its authenticity could not be verified. Iraqi officials said the uniformed men were from Islamic State, not the army.
"The video belongs to the Daesh mobs and it becomes clear through analyzing the scene, which shows individuals who wear non-military clothes, different from that worn by the Iraqi army," Maj. Gen. Najim Jabouri said in a statement released by the Iraqi War Media Office, using an Arabic acronym for the extremists.
He also noted that guns used in the video "are the same guns used by Daesh militants."
The day before, Amnesty International called on the government to investigate reports that fighters wearing Iraqi federal police uniforms tortured and killed people suspected of collaborating with the Sunni Muslim extremists in villages south of Mosul.
Iraqi Prime Minister Haider Abadi rejected the allegations of unlawful killings as "false information."
"The truth is that they were local residents who killed Daesh militants," Abadi said of the deaths.
Belkis Wille, a senior Iraq researcher at Human Rights Watch, advised "extreme caution" when assessing videos like the one that surfaced last week. "We know that Islamic State previously produced fake videos showing Iraqi forces committing abuses," she said.
She noted that the tank in the video did not have any type of flag on it, and no other armored vehicles could be seen in the area. "Also, only one man in the video is wearing an Iraqi uniform," she said.
Still, she said the video should be investigated, and if the men in it prove to be Iraqi forces, authorities should take swift action against them.
"Unfortunately in the battle to retake Fallujah, we've seen multiple instances of abuses perpetrated by pro-government forces against the civilian population," she said of another recent offensive. "There is an extreme concern that this may happen again in Mosul."
Human Rights Watch reported that at least 37 men suspected of being affiliated with Islamic State had been detained by Iraqi and Kurdish forces from checkpoints, villages, screening centers and displaced persons camps around Mosul.
Relatives did not know where most of the men were being held and had not been able to contact them, according to the report, which warned that such conduct "significantly increases the risk of other violations," including torture.
Iraqi soldiers have shown the Los Angeles Times the bodies of several alleged Islamic State militants since the offensive began.
In the village of Gogjali, east of Mosul, an Iraqi special forces commander pointed out the body of a white man he said was a foreign fighter killed while fleeing after a gunfight. The man was unarmed, dressed in civilian clothes and had been shot in the head.
Soldiers elsewhere in Gogjali showed The Times two burned bodies of men they said had tried to assist an Islamic State suicide bomber.
The soldiers said the men were armed with a machine gun, so they shot them with a bazooka, which set the men on fire. The machine gun had vanished. The soldiers said they took it.
The decapitated body that was lying in the street Saturday belonged to a bearded man, who was wearing a brown T-shirt, fingerless gloves, tan pants and combat boots.
When soldiers lifted the blanket that was covering him, the head was next to the body. But there was a pool of blood on the stomach that suggested the head had initially been placed there.
Islamic State militants often stage their victims that way in photographs circulated online.
A piece of string lay by the man's left hand.
Maj. Ahmed Mahmoudi did not want to stay long. There were snipers in the area, he said.
Asked how the man and the seven other Islamic State fighters caught the night before had died, he said, "All of them died because of the clashes," including a suicide bomber who blew himself up.
Was the man in the street beheaded?
"No, it was because of the firing," Mahmoudi said.
What about the string? Had the man been bound or dragged?
"We didn't drag him," Mahmoudi said. "We are not dealing with Daesh as Daesh is dealing with us."
How did the body end up there?
"Okay, we dragged him," he said. "But about his head, it was because of the shooting."
Then the major offered another possible explanation: "You know, animals, dogs, at night…"
Mahmoudi said Iraqi forces had taken a number of Islamic State fighters prisoner during this and past offensives. "We are not here to take revenge or to execute people," he said.
On Thursday, soldiers in Gogjali allowed one such captive, an admitted Islamic State fighter, to talk with The Times and be photographed before taking him away for further questioning. He appeared unharmed. But The Times was not able to speak with the man again or to verify where he had been taken.
Mahmoudi said he and his men captured another militant who was dressed in street clothes and tried to blend with civilians. The man admitted to fighting for Islamic State, he said, and they handed him over to military intelligence.
Mahmoudi said his soldiers come from different sects and were professionally trained.
"We would not cut the head off," he insisted. "Our mission is to free people."