Lawyer David Sheldon said the investigator agreed with military prosecutors that his client, Specialist James Barker, and three other soldiers charged with raping the girl and killing her, her 6-year-old sister and their parents at their home in Mahmudiya, be court martialled.
"The investigating officer recommended that my client and the other three soldiers face a general court martial," Mr Sheldon said.
At a military hearing in Baghdad a month ago, a prosecutor demanded a court martial for Barker, Sergeant Paul Cortez, and Privates First Class Jesse Spielman and Bryan Howard.
The prosecutor said that Abeer al-Janabi, 14, was found shot dead at home along with the other family members. He said some of the accused testified they and other members of their checkpoint unit killed the four and raped the teenager.
The case shocked Iraqis otherwise accustomed to violence and angered the US-backed Iraqi government.
The prosecutor dismissed a defence case that they were "driven nuts" by the strain of combat in an area where insurgents are active.
Mahmudiya is said by US infantrymen to be the "triangle of death" because of the constant deadly attacks by Iraqi insurgents.
Mr Sheldon said he would file objections to the recommendation concerning his client by the close of business on Friday, while the New York Times reported that counsel for the other soldiers are also preparing challenges to the investigator's report.
"The investigating officer denied critical witnesses to Barker and the others accused. He also denied their right to a public hearing," Mr Sheldon said.
A fifth soldier is charged with dereliction of duty.
Former private Steven Green, since discharged from the unit, faces trial for rape and murder in a US civilian court.