The deal also ensures that Watts, who is 33, will consecutively serve each sentence for that charge and the murders of his wife and daughters.
Watts' defense attorneys came to the prosecution weeks ago with proposals for plea deals, according to Rourke, though he doesn't know why they did.
"I was not willing to entertain any further concessions above and beyond removing the death penalty", he said.
'That to me was the most important consideration in deciding how to proceed in this a case.
Watts will be sentenced on November 19 and is facing life in prison.
Rourke said he and another prosecutor visited Shanann Watts' family in North Carolina last month to discuss the odds of a death penalty sentence being carried out.
"That about as firmly as she could have said it to me", Rourke said. The district attorney said he thinks authorities have a partial motive for the killings.
Chris Watts pleaded guilty Tuesday to all nine counts he faced.
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"I know that that will never be fully realized because, obviously, the tragedy that sits before us today is the loss of four attractive lives".
Mr Rourke said he was not willing to dismiss any charges in exchange for the promise of guilty pleas.
While both Rourke and CBI Director John Camper said they hope this provides some closure and sense of justice, they also recognized that nothing that happens in the courtroom can fix what happened in the Watts' Frederick home in August. If it goes as planned, Rourke said he will not fight the release of the autopsy reports for Shanann Watts and her daughters.
Shanann Watts, 34, and her two daughters ages 3 and 4 were reported missing. Watts was arrested shortly before midnight on August 15. Watts initially claimed he killed his wife in an act of passion after finding one daughter dead and Shanann trying to kill the other.
Investigators say Watts killed his family and then dumped their bodies on the property belonging to an oil company.
The girls' bodies were found submerged in an oil tank, and Shanann Watts' body was found in a shallow grave.
"The spotlight that he tried to shine on Shanann - falsely, incorrectly and frankly a flat-out lie - has been corrected", Rourke said.
Under Colorado law, a first-degree murder conviction carries a mandatory life sentence without the possibility of parole.