US Army Specialist Terry Bowman said he "was told by his chain of command what version to give" to investigators, Sgt Irene Cintron of the Army's Criminal Investigation Command said during a teleconference from Iraq on Wednesday.
The US military has been convening a hearing to determine whether the soldiers will be court martialed.
Bowman said he had been ordered to push the men into the Tigris River, Cintron said.
It was not disclosed who gave the order. Three of the soldiers' commanders have received nonjudicial punishments for their roles in the incident. None of those punishments include jail time.
Forced to drown
The soldiers were accused of forcing two Iraqis off a bridge into the Tigris river in January.
The Iraqis had initially been picked up for curfew violation near the Iraqi city of Samarra.
One of the two, Zaydun Maamun Fadhil al-Samarai, drowned while the other - his cousin Marwan al-Samarai - managed to get out of the river.
The case against the four soldiers comes in the wake of revelations about rampant abuse of prisoners in Iraq, Afghanistan and at Guantanamo Bay.
Sgt 1st Class Tracy E. Perkins, 33, 1st Lt Jack M. Saville, 24, and Sgt Reggie Martinez, 24, are charged with involuntary manslaughter in the drowning death of Fadhil al-Samarai, while Bowman, 21, is charged with assault for allegedly pushing the second man into the river at the same time.
Some Iraqis who lost relatives to
US action joined the resistance
Marwan al-Samarai said he tried to help his cousin swim to safety, only to lose his grip as the soldiers watched and laughed.
"They were behaving like they were watching a comedy on stage," he told the AP several weeks ago.
Perkins, Martinez and Bowman appeared at Wednesday's Article 32 hearing, which is similar to a civilian grand jury hearing and will determine whether the soldiers are court martialed.
Saville's hearing is tentatively scheduled for 9 Sept. The four soldiers face between 5 1/2 and 26 1/2 years in prison if they are tried and convicted.
The drowning incident almost went untold until a popular online blog - healingiraq.blogspot.com - posted a public appeal from al-Samarai's mother calling for an investigation into her son's death.
She claimed the two cousins had been hauling toilet spare parts from Baghdad to Samarra when their truck broke down on 3 January.
After repairing it, they were stopped at a US checkpoint at 22:45, 15 minutes before the beginning of the nightly curfew.
The two Iraqis were allowed to pass but were then followed by a "mechanised vehicle". They were stopped, forced out of their truck, handcuffed and taken to a bridge overlooking the Tharthar Dam where they were then allegedly forced to jump into the Tigris river.
The mother wrote an open letter to US President George Bush and British Prime Minister Tony Blair pleading for an investigation into her son's death.
An investigation was opened by US occupation forces on 8 January.
According to Cintron, investigators learned of the death in an e-mail from the victim's family. She said she met with Marwan al-Samarai on 16 January.
Marwan al-Samarai said he watched the soldiers push his cousin into the water and then he was pushed in, Cintron testified. He said he could hear his cousin screaming.
US Private Lynndie England faces
charges of abuse and toture
"He said it was eight metres (26 feet) deep and at no point did he feel the bottom of the river," she said. She described the drop from the bridge as 3-3.6 meters (10-12 feet).
After he got out on the bank, al-Samarai said he could hear the soldiers above laughing as they drove away. He said he went back to a checkpoint "soaking wet from the river" and reported what happened, Cintron said.
His cousin's body was found three to five kilometres downriver.
Martinez initially told investigators neither he nor anyone in his platoon pushed anyone in the river, Cintron said. A week later, on 23 January, Martinez said he had gone to the river's edge with the men, "kicked one in the butt" but the man jumped in on his own.
However, Sgt Alexis Rincon, a member of the patrol that night, testified the soldiers forced the men to jump and that Martinez leveled a rifle at one of them. Rincon said the man hesitated, but jumped after the second Iraqi said something to him in Arabic.
None of the soldiers thought the men were in danger because one quickly made it to shore, Rincon said. He said he would not have left the scene had he known one of the men was drowning, but asked if he would have gone to the man's aid, Rincon replied: "I don't know about jumping in and saving him."
The soldiers' commanders, Lt Col Nathan Sassaman, Maj Robert Gwinner and Capt Matthew Cunningham, were punished last spring under Article 15, which allows punishments without a court proceeding or public record.
The soldiers are assigned to Fort Carson's 3rd Brigade Combat Team. The brigade is part of the 4th Infantry Division, based at Fort Hood, Texas.