Steven Dale Green in a mug shot at the Mecklenburg County jail in Charlotte, N.C., in 2006. File/AP

A former U.S. soldier died Saturday, two days after he apparently attempted suicide in the prison where he was serving multiple life sentences for raping an Iraqi teenager and killing the girl, her parents and her sister in March 2006 during his deployment in Iraq.

The case was the first prosecuted under a new law governing military cases in civilian courts.

Steven Dale Green, 28, of Midland, Texas, was found unresponsive in his cell last week at the federal penitentiary in Tucson, Ariz., said Federal Bureau of Prisons spokesman John Stahley. Green's death was not announced until Tuesday because prison staff was off for the long Presidents' Day weekend, Stahley said.

Stahley said Green's death is being investigated as a suicide.

Green was a 19-year-old private in the 101st Airborne Division based at Fort Campbell, Ky., when he was deployed to Iraq.