Marine Staff Sgt. Giorgio Kirylo said he watched as Special Operations Chief Edward Gallagher tried to save the 17-year-old captive and never saw him use his hunting knife on him.
Gallagher is accused of fatally stabbing the adolescent captive while he was under his care in Iraq in 2017 and to shooting civilians. He has pleaded not guilty to murder and attempted murder.
After the boy died, Kirylo said he moved the corpse to pose for a "cool guy trophy" photo because it was the platoon's first Islamic State prisoner captured from their battlefield.
He said he lifted up the teen's body to put the corpse's head on his foot when a bandage on his neck popped up. Kirylo testified that he saw no stab wounds under the dressing.
He said each member of the platoon took turns posing with the body, and no one was upset.
He said the platoon members took turns taking photos with the body because they were excited that they had coordinated an air strike with Iraqi troops that had killed Islamic State fighters including the captive's commander.
"This was our unofficial war trophy," the Marine Raider said.
Kirylo statements contradict those of SEALs who testified that Gallagher, who was on his eighth deployment, stabbed the prisoner.
Two of the SEALs testified that they witnessed the stabbing, but one - Corey Scott - said he was the one who ultimately killed the militant by plugging his breathing tube with his thumb as an act of mercy.
The Navy said it is reviewing Scott's statements following his stunning testimony last week and it may press perjury charges.
Kirylo said he was close friends with several SEALs who reported the stabbing, but he now considers them liars.
Many of the SEALs who testified said their names were made public and spread on social media because they came out against the chief. One indicated that taking the stand has jeopardized his career.
Kirylo described the team as full of young SEALs who complained about Gallagher putting them close to the front lines and stealing their care packages. He described Gallagher as an "old school" SEAL whose aggressive style was forcing ISIS fighters to come "out of the woodwork into a shooting gallery."
He said the night after the militant died that one of the SEALs encouraged the team to delete the photos, telling them it was a war crime. Kirylo told them it was not.
The military jury on Thursday also saw videotaped testimony taken June 3 from the Iraqi general who handed over the war prisoner after he was injured in the airstrike outside Mosul.
Gen. Abbas al-Jubouri, whose forces were partnered with U.S. troops, testified that he ordered his troops to hand over the militant to Gallagher and his fellow SEALs because he wanted to keep him alive for interrogation later.
The general said during defense questioning that he did not see Gallagher harm the captive in any way - and if he had, he would have spoken up.
"If he did any mistake with this kid, or if anyone had from the Navy SEALs, I would have stopped them," al-Jubouri said.
When he was shown photos of the dead militant with bandages around his neck and tubes in his chest, al-Jubouri said he did not recall seeing the boy with all that.
Weber reported from Los Angeles.
Corrects spelling of Marine to Giorgio Kirylo instead of Georgio, and that two of the SEALs testified that they witnessed the stabbing.
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