(CNN) - Chris Watts kept changing stories after his pregnant wife and two daughters disappeared from their Colorado town in mid-August.
And even though he pleaded guilty Tuesday to first-degree murder and other charges in their deaths, it's still not known publicly what happened to 34-year-old Shanann Watts and their daughters, Bella, 4, and Celeste, 3.
When Shanann Watts and her two little girls went missing from their home in Frederick, about 30 miles north of Denver, Chris Watts first told news cameras that they had gone to a friend's house.
Later, as people searched for his missing family, Watts told CNN affiliate KMGH he had had an emotional conversation with his wife, Shanann, the last time he saw her. He gave no details about that talk, only saying, "I just want them back."
He initially told law enforcement that on the morning of August 13 he told his wife he wanted to separate and then left for work.
Police said that after they found the bodies three days later, Chris Watts, 33, then said he strangled his wife in a rage after he saw her on a baby monitor choking the couple's 3-year-old daughter.
Shanann Watts' body was found in a shallow grave, and the bodies of Bella and Celeste were discovered in commercial oil tanks at a company where Chris Watts had worked. Autopsy results have not been released.
In court Tuesday, Watts took responsibility for all three deaths, pleading guilty to nine charges, including first-degree murder and unlawful termination of a pregnancy.
In exchange for his pleas, Watts will not face the death penalty, according to Weld County District Attorney Michael Rourke.
Authorities aren't entirely sure why the husband and father killed his wife and their daughters. Rourke told reporters that authorities have what he called a partial motive, and said he would offer more thoughts after Watts is sentenced.
The district attorney said lawyers for Chris Watts reached out several weeks ago about making a deal.
Rourke said he flew to North Carolina to discuss the possible agreement with relatives of Shanann Watts and they signed off on removing the death penalty option.
"(Her mother) said, 'He made the choice to take those lives,'" the district attorney said. "'I do not want to be in a position of making the choice to take his.'"
Shanann Watts' mother, father and brother were in court Tuesday but chose not to speak to the media.
Rourke said he was not willing to dismiss any charges in exchange for the promise of guilty pleas.
"I was not willing to entertain any further concessions above and beyond removing the death penalty," he said.
He said there were no winners in reaching a plea deal.
"I can only say that I hope there is a sense of closure (for family members of the deceased)," Rourke told reporters. "I know that that will never be fully realized because, obviously, the tragedy that sits before us today is the loss of four beautiful lives (including Shanann's unborn baby)."
Prosecutors will ask for a judge to sentence Watts to life in prison, instead of seeking the death penalty.
Investigators never believed Watts' stories, Rourke said.
Watts will be sentenced November 19. Rourke said the plea deal asks the judge to make the first-degree murder sentences consecutive.
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