By the time officials arrested suspects months later, two more Afghans were dead.
And much to Christopher Winfield's horror, his son Adam was among the five Fort Lewis-based soldiers charged in the killings.
The elder Winfield told The Associated Press in an exclusive interview that his son did not kill the unarmed man and would never have been in the situation if the Army had investigated the warnings he says he passed along to Fort Lewis.
An Army spokeswoman at the base said she could not comment on whether they received such a tip or if so, whether it was acted on.
But the new details about Winfield's efforts to alert the Army and his son's pleas raised questions about the Army's handling of the case and its system for allowing soldiers to report misconduct by their colleagues.
The soldiers have been accused of conspiracy and premeditated murder. The highest ranking is Staff Sgt. Calvin Gibbs, who, along with Cpl. Jeremy Morlock, are accused of taking part in all three killings.
Pfc. Andrew Holmes is charged with murder in the first killing, and Spc. Michael Wagnon is charged in another. Both deny the charges.
Winfield is charged with murder in the final killing, and his attorney, Eric Montalvo, insists he was ordered to shoot after Gibbs hit the civilian with a grenade. Winfield deliberately shot high and missed, he said.
Gibbs has denied the charges. His attorney, Phillip Stackhouse, said his client maintains that the shootings were "appropriate engagements" and denies involvement in any conspiracy to kill civilians.
The soldiers, all assigned to the 5th Stryker Brigade, deployed in July 2009 and were stationed at a base in Kandahar Province.
The AP reviewed witness and defendant statements as well as documents filed with an Army magistrate for this report.
Gibbs, 25, of Billings, Mont., arrived in the unit late last year and soon began discussing how easy it would be to kill civilians, some in the platoon told Army investigators. He and Cpl. Jeremy Morlock, 22, planned "scenarios" in which they could carry out such killings, they said.
Morlock, of Wasilla, Alaska, gave investigators extensive statements describing the plot.
Morlock's lawyer did not immediately return calls and e-mails from the AP, but previously told The Seattle Times that the statements were made under the influence of prescription drugs to treat traumatic brain injuries from explosions and should be suppressed as evidence.
In each of the killings, Morlock said, he and Gibbs planned and initiated the attack and enlisted one other soldier to participate.
The first indication for Christopher Winfield and his wife, Emma, that something was amiss came Jan. 15, the day of the first killing.
"I'm not sure what to do about something that happened out here but I need to be secretive about this," their son wrote them in a Facebook message. The couple gave the AP copies of the Facebook messages, Internet chats and their phone records.
Winfield, 22, of Cape Coral, Fla., didn't immediately provide more details, and over the next month he had little contact with his parents. They said they checked constantly to see if he was online.
On Feb. 14, he told his parents what happened in a lengthy Internet chat: Members of his unit on patrol had killed "some innocent guy about my age just farming." He said he did not witness the killing.
But, he wrote, those involved told him about it and urged him to "get one of my own."
He said that virtually everyone in the platoon was aware of what was going on, but no one seemed to object.
"If you talk to anyone on my behalf, I have proof that they are planning another one in the form of an AK-47 they want to drop on a guy."
He added that he didn't know whom to trust and feared for his safety if his comrades learned he was talking to authorities.
"Should I do the right thing and put myself in danger for it. Or just shut up and deal with it," he wrote his parents. "There are no more good men left here. It eats away at my conscience everyday."
In statements to investigators, at least three platoon members said Gibbs directly threatened Winfield. Morlock added that Gibbs devised "scenarios" for Winfield's death, one of which involved Gibbs dropping heavy weights on him as he was working out.
Gibbs accosted Winfield as he was on his way to speak with a chaplain and warned him to keep quiet, Montalvo said.
Soldiers serving in a combat theater typically would report crimes up the chain of command, to military investigators or chaplains, to members of the Defense Department inspector general's office, or even to another unit if their own commanders are involved.
One soldier, Pfc. Justin A. Stoner, who reported hashish smoking in the unit, said he was beaten by several platoon members. Gibbs and Morlock then paid him a visit, with Gibbs rolling out on the floor a set of severed fingers, he told investigators.
Morlock told him that "if I don't want to end up like that guy ... shut the hell up."
Winfield asked his parents to call an Army hotline because he didn't want anyone to overhear him using the phone.
His father, a Marine veteran, was shocked, and made five calls to military officials that day, his phone records show.
He said he left a message on a Defense Department hotline and called four numbers at Fort Lewis. He said he spoke with an on-duty sergeant and left a message at an Army Criminal Investigations Division office before reaching the base's command center.
In that call, an official told him that if his son wasn't willing to come forward while deployed, there was nothing the base could do, Winfield recalled in interviews with the AP and in a sworn statement to Army investigators.
The official suggested the soldier keep his head down until his deployment ended and investigators could look into his claims, he said.
The elder Winfield told AP he regrets not writing down the identities of those he spoke with. He said he did not give any of them Gibbs' name, but did identify his son. He said one of his son's sergeants had been involved in a civilian's murder and was planning more.
His son soon expressed concern about what would happen if Army officials stateside began making inquiries, and asked his dad to back off. The elder Winfield said he complied.
A week later, the second killing occurred. On May 2, the third killing took place.
The killings eventually came to light when the soldier who had reported the drug use told investigators that Morlock "had three prior kills that none of which I believe were actually justified."
Preliminary hearings in the case are expected to begin this fall.
Associated Press writer Pauline Jelinek contributed from Washington, D.C.