The investigation of four men from the famed "Rakkasans" -- the 3rd Combat Brigade of the 101st Airborne Division -- is expected to focus a critical light on the US military's controversial and opaque rules of engagement in Iraq.
Civilian defence lawyers have said orders from Rakkasans’ commander Colonel Michael Steele called for troops to "kill all military age males" during a raid on May 9 on a suspected Al-Qaeda base.
Colonel Steele is a controversial figure following his leading role in a bungled 1993 raid in the Somali capital Mogadishu, made famous by the book and film "Blackhawk Down", in which 18 US soldiers and hundreds of Somalis died.
Private Bradley Mason was testifying on the second day of a pre-trial hearing at Camp Speicher near Tikrit, in central Iraq, to decide whether to charge the four with premeditated murder.
Private Mason testified that he had been with Private Corey Clagett, Staff Sergeant Raymond Girouard, Specialist William Hunsaker and Specialist Juston Graber during the operation.
The US troops arrived at a suspect house by helicopter, shot one man at the window and captured three more hiding inside with two women, he said. A pistol, an AK-47 rifle, ammunition and gun parts were found in the house.
Private Mason said Sgt Girouard told him Specialists Clagett and Hunsacker were going to kill the prisoners. "They just smiled," he added, describing their reaction to the alleged instruction.
"I told him that I'm not down with it. It's murder," Private Mason said.
The 20-year-old said he then heard gunshots, and left the house to find the detainees dead.
Afterwards, he said, Specialists Girouard and Hunsaker threatened to kill him if he reported what had happened.
According to Private Mason, Specialist Girouard said: "If you say anything, I'll kill you."
Under cross-examination from Specialist Clagett's lawyer Paul Bergrin, Private Mason said Colonel Steele had ordered before the mission that if troops encountered Iraqi men they were to "kill all of them."
The defendants have told military investigators the detainees were shot after they broke the plastic cuffs binding their hands, assaulted their captors and attempted to flee.
Colonel Steele has signed a document declaring his intention to refuse to testify to avoid incriminating himself, lawyers said, on the first of what is expected to be a four-day hearing.
Three more potential defence witnesses -- Captain Daniel Hart, Sergeant Eric Geressey and Sergeant Leonel Lemus -- have also refused to testify
Under military law, the hearing will decide whether the men should face a full court martial. Charges are expected to include murder, conspiracy, making threats and obstructing justice.
The Rakkasans' motto, which is emblazoned in front of brigade headquarters in Tikrit, is: "We give the enemy the maximum opportunity to die for his country."