Atlanta, Georgia (CNN) -- The Army may have known months ago about serious misconduct -- including the apparently unprovoked murder of a civilian -- by members of a platoon in Afghanistan but failed to act on it before at least one other murder occurred.
In an interrogation tape obtained by CNN, Spc. Adam Winfield, 21, tells an Army investigator in May that he told his father in February that he feared for his life after hearing that others within his platoon had murdered an Afghan civilian. He feared that his comrades -- members of the 5th Stryker Brigade -- were hunting for other victims, he said.
But a warning to the Army by Winfield's father may have been ignored.
In an Army interrogation tape obtained by CNN, Winfield describes hearing members of his platoon bragging in January 2010 about having killed a man and then made the killing look like it was carried out in self-defense.
"They said the guy threw the grenade at them, and that's how the grenade went off, and then they shot him," Winfield told an Army investigator.
Asked what really happened, Winfield said on the tape that others in his platoon -- Cpl. Jeremy Morlock and Staff Sgt. Calvin Gibbs -- were involved. They are among five soldiers facing premeditated murder charges for the killings of three Afghan civilians between January and May of this year. Seven other soldiers face lesser charges.
"Morlock and [another soldier] were planning it for about a week or so," Winfield said. "Sergeant Gibbs, you know, put it into their heads that they could get away with doing these things."
U.S. Army charging papers describe Gibbs as the ringleader in Afghanistan of a band of rogue U.S. infantrymen, some of whom were routinely smoking hashish laced with opium.
According to statements from the accused, some of the men collected body parts from the victims -- including fingers and bones -- and posed for pictures with their corpses.
Gibbs allegedly encouraged his men to join him in killing civilians, and ordered them to keep their activities secret, an order that Winfield paid attention to.
"I take that man very seriously," Winfield told the investigator in the interrogation tape. "He likes to kill things. He is pretty much evil incarnate. I mean, I have never met a man who can go from one minute joking around, then mindless killings. I mean, he likes to kill things."
Asked how he thought Gibbs would have responded had Winfield refused to participate in the killing, Winfield said, "I think -- one, he wouldn't have kept me in the loop on things and, if they had thought I had ratted, they would have come after me."
After the killing of Afghan civilian Gul Mudin in January, Winfield called his father -- himself a retired soldier -- and told him what had happened, according to Adam Winfield's attorney, Eric Montalvo.
When the elder Winfield told the Army inspector general in February what his son had told him, he was told that nothing could be done and that his son should lay low, the attorney said.
When CNN called the Army to ask about the call, an Army spokesman said an investigation is now under way.
Winfield himself is charged with premeditated murder in the killing of another Afghan civilian in May.
His client is not guilty of premeditated murder, Montalvo said, despite the existence of an Army interrogation tape on which Winfield appears to implicate himself and other members of his platoon, including Morlock and Gibbs.
On the tape, Winfield describes the fatal encounter with the Afghan civilian for which he is charged. "He seemed friendly, he didn't seem to have any animosity towards us," Winfield says. "Brought him out, sitting in the ditch. Me and Morlock were behind the berm ... Sergeant Gibbs said, 'This is how it's going to go down. You're going to shoot your weapons, yell "grenade," I'm going to throw this grenade. After it goes off, I'm going to drop this grenade next to him. That's it.' So he had two grenades on him."
Winfield: "So, we're laying there and Morlock told me to shoot, so started shooting. Yelled 'grenade,' grenade blew up and that was that."
Investigator: "Who gave you the order to shoot?"
Winfield: "Morlock ... and we fired once he said 'Shoot.' Grenade blew up. Sergeant Gibbs threw the grenade; it blew up and then he came over, shot the man probably two more times in the head."
Winfield said that Gibbs told him after the killing that he was part of the group.
Investigator: "Did he ever hold against you that you killed a man?"
Winfield: "No, he told me I was a made man after that."
Gibbs' attorney did not respond to CNN's request for comment.
Morlock's attorney told CNN that his client suffered from brain injuries suffered in IED attacks and was on Army-prescribed medication that impaired his judgment.
The Pentagon has not commented on any of the cases. Instead, it sent CNN a statement about the videos: "Disclosure of the video recordings to the public at this juncture is troubling because it could adversely affect the fair and just administration of the military justice process," the statement says.
CNN's Kathleen Johnston, Courtney Yager, Scott Zamost, Todd Schwarzschild, Drew Griffin and Tom Watkins contributed to this story.