An Iraqi army doctor testified of his horror upon seeing the dead teenager at her home in Mahmudiyah, south of Baghdad.
The girl was allegedly raped and killed, along with her parents and five year old sister, and the house set on fire.
The doctor said the girl's upper torso and head were burned and she had a single bullet wound under her left eye.
Sergeant Paul Cortez and Anthony Yribe, Specialist James Barker, Private Jesse Spielman and Private Bryan Howard could face the death penalty if found guilty of taking part in the attack.
A sixth suspect, Steven Green, has already been discharged from the army because of a "personality disorder" and will be tried separately in the United States in a civilian federal court.
Lieutenant Colonel Thomas Kunk, commander of the Battalion to which the accused soldiers belong, also testified.
Kunk said at first he disbelieved the allegations, but had questioned some of the accused, including Yribe who had been sent to the scene of the deaths to conduct an investigation before coming under suspicion himself.
Kunk also recalled questioning Barker, who he described as "very flippant, very confident, and more than willing to answer the questions I had."
Kunk also recalled Green saying that "all Iraqis are bad people."
Prosecutors at Camp Liberty, a US base at Baghdad airport, will allege that Green shot dead the girl's father, mother and sister then raped the teenager, killed her and set her house on fire to cover his tracks.
An affidavit accuses Green's comrades of helping him plan, carry out and cover up the slaying.
The case is the latest in a series of high-profile scandals that has tarnished the reputation of US troops in Iraq.
US marines are said to have gone on a rampage in November last year, after a bomb killed a comrade.
They are accused of killing 24 civilians, including 10 women and children.
Two US soldiers have also been charged with voluntary manslaughter over the killing of an unarmed Iraqi civilian outside his home four months ago.
Last month, seven marines and a sailor were charged with premeditated murder and kidnapping over the killing of an Iraqi in the town of Hamdania near Baghdad.
Four more men will this week face a hearing into allegations that they shot dead three Iraqi prisoners in cold blood after receiving an order to "kill all military-age males."