Dwight Warren, the investigator in the case, said in a report issued on Sunday: "Reasonable grounds exist to believe that each of the accused committed the offence for which he is charged."
The report was given to lawyers in the case and obtained by The Associated Press from David Sheldon, the lawyer of one of the defendants.
The four soldiers of the 101st Airborne Division are charged with raping the girl in her family's home in Mahmoudiya, about 30 kilometers (20 miles) south of Baghdad, then killing her along with her parents and her younger sister.
Military prosecutors say the four set the teenager's body on fire in order to hide their crime. If found guilty they could face the death penalty.
Another soldier is accused of failing to report the attack but is not alleged to have been a direct participant.
One soldier who was said to have allegedly planned the attack, Steven D. Green, was discharged from the army due to a "personality disorder" before the allegations became known.
He was arrested in June shortly after the allegations became known. He has pleaded not guilty to rape and murder charges and is being held in a civilian court in the United States.
Mahmoudiya, where the incident occurred, is an extremely violent region of Iraq within an area known as the 'triangle of death' for the numerous attacks by insurgents, and lawyers are expected to use extreme combat stress as a defence.
Sheldon said the soldiers were essentially abandoned by a military command that did not give them the support that they needed, either in the field or in the courtroom.
Sheldon told the Associated Press: "I'm not surprised given the events in this case. It was apparent that neither Spc. Barker nor any of the other soldiers were going to get a fair hearing."
He said that he would be filing an objection within the five days allowed after he received the recommendation from the investigator.
Testimony in early August during the soldiers' Article 32 hearing - similar to a civilian grand jury hearing - painted a picture of a unit that was almost constantly on edge from repeated attacks and demoralised by the loss of fellow soldiers.
Sheldon said: "Each one of these soldiers had experienced extreme combat distress."
In his report, the army investigator did not make a recommendation on whether the accused should face the death penalty, but he outlined a number of aggravating factors that could be considered.
The allegations of rape and murder have bolstered Iraqi accusations of misconduct by soldiers, including illegal killings, beatings and inhuman treatment.
The allegations have increased the mistrust and resentment among Iraqis of the American military and increased calls for their withdrawal.