Chris Watts, 33, has pleaded guilty to murdering his pregnant wife and their children in a case that shocked the world.
The bodies of the girls, Bella, 4, and Celeste, 3, were found in oil barrels, while wife Shanann's remains were discovered in a shallow grave nearby.
Shanann Watts was 15 weeks pregnant when she died, according to court records.
Rourke said he hoped the agreement would bring some measure of closure for the family, who were present during the court hearing and the press conference.
"I know that that will never be fully realized because, obviously, the tragedy that sits before us today is the loss of four lovely lives".
The plea deal includes a sentencing agreement that Christopher Watts will be sentenced to spend the rest of his life in prison, Weld County District Attorney Michael Rourke said. Rourke told reporters that authorities have what he called a partial motive, and said he would offer more thoughts after Watts is sentenced. He did not look at member of Shanann Watts' family.
In August, after Shanann and the two girls were reported missing, Watts appeared on local television pleading for their safe return.
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Chris Watts kept changing stories after his pregnant wife and two daughters disappeared from their Colorado town in mid-August.
Rourke said he described "extraordinary delays" in carrying out the death penalty in Colorado since Gov. John Hickenlooper's 2013 decision to block the execution of Nathan Dunlap.
At the time of her disappearance in August, according to an arrest affidavit, Christopher Watts initially told police officers he had just informed his wife that he wanted a separation. He added that law enforcement never believed Watts' defense that Shanann Watts had strangled her daughters to death, sending him into a rage that led him to strangle his wife to death.
He told officers he loaded his wife and children's bodies into the back seat of his truck and drove them to an oil and natural gas company he worked for.
"We charged him with what we truly believed he had done", Rourke said.
"What I can tell you most affirmatively today, by what happened in the courtroom, is the spotlight that he tried to shine on Shanann falsely, incorrectly and frankly a flat out lie has been corrected", Rourke said.
Watts will be sentenced November 19. The spotlight shines squarely on him, ' he said.