Medical examiners said the cause of Steven Green's death was suicide by hanging, CNN reported.
The 28-year-old died Saturday.
In 2009, Green was found guilty in U.S. District Court of raping a 14-year-old Iraqi girl and killing her, her parents and her 6-year-old sister in Iraq.
Green was sentenced to multiple life prison terms and apologized for his actions at a victim impact hearing.
"I helped to destroy a family and end the lives of four of my fellow human beings, and I wish that I could take it back, but I cannot," Green said, reading a statement at a victim impact hearing. "And, as inadequate as this apology is, it is all I can give you."
He was the last of five soldiers in the 101st Airborne Division to be convicted for the crimes and covering them up.