The medic, who was not named, said that when he entered the house in Mahmudiya on March 12, he found 14-year-old Abeer Qasim Hamza al-Janabi naked with her legs spread and burned from the waist up, with a single bullet wound beneath her left eye.
He also told the hearings on Sunday that he had found her six-year-old sister in an adjacent room with the back of her head blown out, and the bodies of both parents riddled with bullets.
Four serving soldiers - Sergeant Paul Cortez, Specialist James Barker, Private Jesse Spielman and Private Bryan Howard - are charged with conspiring to rape the teenager.
They could face the death penalty if found guilty.
The fifth soldier, Sergeant Anthony Yribe, is charged with failing to report the attack but is not alleged to have taken part.
The Article 32 hearing will decide if there is enough evidence to start a court martial for the five soldiers.
A former soldier from the same unit has already pleaded not guilty to the rape and murder in a civilian court.
Steven Green had been discharged from the army due to a "personality disorder" and was arrested in North Carolina in June.
Al-Maliki demands an independent
investigation into the case
According to an FBI affidavit, the soldiers drank alcohol before they left their checkpoint, changed clothes and headed to the victims' house, about 200m away.
According to prosecutors, Green shot dead Abeer's father, mother and her sister at the family home in Mahmudiya, near Baghdad. He then raped the teenager and killed her, and tried to burn the bodies and house to conceal evidence of the crime.
Prosecutors say a second soldier also raped Abeer. The New York Times reported on Saturday that it now appeared at least three soldiers raped her, according to a legal memo filed by a military magistrate.
David Sheldon, a lawyer for one of the soldiers, has said the stressful environment in the Mahmudiya area contributed to the soldiers' behaviour.
US officials have assured the Iraqi government that the soldiers will be punished if convicted.
Nuri al-Maliki, the Iraqi prime minister, has demanded an independent investigation into the case.
Parts of the inquiry - which is expected to last several days - will be held in secret, including testimony from Iraqi witnesses. The restriction was imposed to protect witnesses amid fears they could be seen as collaborating with the Americans.
The Iraqi authorities identified the rape victim as Abeer Qassim Hamza. The other victims were her father, Qassim Hamza; her mother, Fikhriya Taha; and her sister, Hadeel Qassim Hamza.
The case is one of five serious crimes allegedly carried out by US soldiers that are being investigated by the military.