Prosecutors sought the death penalty for Steven Green, but jurors in federal court in Kentucky were unable to agree on the sentence, The New York Times reported.
Green, a private first class, was part of a unit in the 101st Airborne Division that was sent to the Triangle of Death south of Baghdad in the fall of 2005.
The killings took place in March 2006. Investigators Green and other soldiers broke into Abeer Qassim Hamza al-Janabi's house in Mahmudiya. Others in the group testified Green killed the teen's parents and sister and shot her in the head.
Four other soldiers pleaded guilty and were sentenced to prison.
Green, 24, of Midland, Texas, was arrested in 2006, shortly after his honorable discharge from the Army. He was convicted May 7.
Testimony during the penalty phase focused on Green's mental state after months under fire. His lawyers argued that his unit had weak leadership and that he should have received treatment.