"He pulled out one of his grenades, an American grenade, you know, popped it, throws it, tells me where to go to whack this guy, kill this guy, kill this guy," Morlock told the investigators.
Morlock said Sergeant Gibbs carried a Russian grenade to throw next to the body of the dead Afghan, to make it seem he was about to attack the American soldiers.
The corporal said he opened fire as directed, fearful of not following Gibbs' orders.
"It's definitely not the right thing to do," Morlock told the investigators. "But I mean, when you got a squad leader bringing you into that, that type of real, that mindset, and he believes that you're on board with that, there's definitely no way you wanted him to think otherwise."
The investigator asked Morlock, "Because you felt maybe the next shot might be coming your way?"
"You never know. Exactly," answered Morlock. "I mean Gibbs talked about how easy it is, people disappear on the battlefield all the time."
In addition to murder, the Army's charging documents allege rampant drug use in Morlock's unit, as well as the dismemberment of dead Afghan civilians.
In the video, Morlock describes how Sergeant Gibbs allegedly collected the fingers of some of his Afghan victims.
"It's his thing now," said Morlock. "I don't know, his crazy stuff. War trophies, whatever."
Morlock said Gibbs boasted of carrying out similar murders in Iraq but was never caught and threatened the men in his unit with harm if they refused to participate or revealed what was happening.
"If Gibbs knew that I was sitting in front of this camera right now, there's no doubt in my mind that he'd f---ing take me out if he had to," Morlock told the Army investigators.