The US military's rules of engagement will go on trial after the four soldiers, who said they were operating under orders to "kill military-age males", shot dead three Iraqi civilians.
The soldiers are charged with premeditated murder of three Iraqis, who were taken prisoner in a raid on an island in the province of Salah al-Din.
The island was allegedly used as an al-Qaeda training camp, defence lawyers said.
Defence lawyers for two of the soldiers have said that the Iraqis broke their plastic zip tie handcuffs, attacked their clients and then fled.
"The three guys were running for this berm and that's where they were shot," said Michael Waddington, a civilian lawyer for Specialist William Hunsaker, one of the soldiers who opened fire.
The Article 32 investigation, a military version of a grand jury hearing to determine if there is enough evidence to warrant a trial or court martial, is scheduled to open on Tuesday at a US military base near Tikrit, Iraq.
The military has provided few details except to say that the four conspired to murder the Iraqis, shot them to death and then two of them threatened to kill a fifth soldier if he talked about it.
Under investigation are Hunsaker, Staff Sergeant Raymond Girouard, Private Corey Clagett and Specialist Juston Graber.
Hunsaker and Clagett were believed to be the only two who fired on the Iraqis.
According to the New York Times, the fifth soldier, Sergeant Lemuel Lemus signed a sworn statement that he witnessed the four plotting to kill the Iraqis after they had been captured and then cover it up.
The Times said Lemus told investigators that another sergeant told Girouard over the radio: "The detainees should have been killed."
First Sergeant Eric Geressy denied having made the statement, and said he did not intend to plant the idea that they should be executed, the newspaper said.
But Lemus said Girouard then gathered Lemus and three other soldiers in a house and, in a low voice and talking with his hands, made it clear he was going to kill the three Iraqis, the Times said.
Worried about the impact of civilian deaths on public support, military commanders say they are paying close attention to what are called "escalation of force" incidents.
The rash of civilian killings has drawn greater scrutiny of military rules and procedures that critics say can vary widely and often appear haphazard, inconsistent and confusing, if not improper.
"There are theatre rules of engagement, there are mission rules of engagement. They can overlap. They can change," said John Sifton, a researcher with Human Rights Watch.
"They can have rules of engagement for patrols, rules for a raid, you can switch in the middle of an operation because something happens. A commander can give orders which will supersede rules of engagement," he said.
"Confusion is bad, because people make wrong decisions, and then later on when they've done something wrong, it allows them to get off the hook, saying they were confused, and thought there was authorisation," he said.
Troops have been given refresher training on military "values" and standards while courses in counter-insurgency warfare for commanders stress the importance of protecting civilians in a complex battlefield.
Lieutenant-General Peter Chiarelli, the commander of foreign forces in Iraq, told reporters in Baghdad recently: "We have people who were on the fence or supported us who in the last two years or three years have in fact decided to strike out against us.
"And you have to ask: Why is that? And I would argue in many instances we are our own worst enemy."