GREELEY, Colorado – Christopher Watts cried, shook and looked on his lap when a judge told him that he would spend the rest of his life in jail for the murders of his pregnant wife and their two Girls.

But he spoke only once at the sentencing hearing, it was to deny the opportunity to make a statement and perhaps answer the question that was gnawing at his parents, his neighbors, Denver, in the suburbs, and others: why?

"How could an apparently normal husband and father annihilate his entire family? Attorney Michael Rourke said at Monday's hearing. "These are questions that only an individual in this audience room – or on this planet – knows about."

The court proceedings revealed a wealth of evidence that prosecutors planned to use against Watts if the case was decided. But without any information from Watts, prosecutors said they could only guess that the killings were meant to give him a "fresh start" with a new girlfriend.

With the approval of Shanann Watts' family, prosecutors agreed not to seek the death penalty in exchange for a guilty plea forcing the 33-year-old to serve life in prison without the possibility of parole.

Judge Marcelo Kopcow called this crime "the most inhuman and vicious crime" in 17 years

After strangling his wife with his hands and smothering Bella, 4, and Celeste, 3, last August, Watts loaded their bodies into his truck. GPS data showed that he was going to a rural oil yard owned by his employer, Rourke said.

He spent a normal day at work and frequently sent messages to his girlfriend, even as the police began investigating the disappearance of his family. He phoned a real estate agent about the sale of the family home and told the girls school that they would not enlist as planned.

Within days, he was accused of killing his family.

The girls' bodies were found immersed in separate oil tanks on a property belonging to her employer. His wife's body was found in a shallow grave nearby.

While the prosecutor detailed the wounds at Monday's hearing, Shanann Watts' father, Frank Rzucek, leaned forward, gasping for air.

Christopher Watts' parents were allowed to speak at the hearing as grandparents of the girls. Cindy and Ronnie Watts urged their son to use his reprieve after a death sentence to make a full confession.

"We hope he embraces this moment," said the couple's lawyer.

Rourke doubts that this moment will come one day.

"These are the questions that will always haunt all those who participated in the investigation," he said. "I do not think there will ever be a satisfactory answer for anyone."

The killings drew national media attention and became the focus of crime blogs and online video channels, showing dozens of family photos and videos that Shanann Watts shared on social media. and showing the smiling family.

Prosecutors said the footage belied Christopher Watts' secret case.

On August 13, a friend asked the police to check if Shanann Watts was unable to reach her. She feared more and more that the future mother had missed a doctor's appointment.

The investigators quickly became suspicious of Watts, who had not been able to say that his family had disappeared from a tightly-secure home in an occupied subdivision.

Meanwhile, Watts met with local television reporters from the front steps of the family home in Frederick, a small town on the plain north of Denver, where drilling rigs and oil wells Surrounding subdivisions in full swing.

He pleaded for the safe return of his family, telling reporters that their home was empty without Bella and Celeste watching cartoons or running for him at the door.

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Colleen Slevin in Denver contributed.