GREELEY, Colo. (Reuters) - A Colorado man was sentenced to life without parole on Monday for strangling his pregnant wife, smothering their two young daughters and dumping their bodies in an oil field so he could pursue a romantic affair.

Christopher Watts, 33, was spared the death penalty at the urging of family members of his wife, Shanann, 34, who was 15 weeks pregnant, and their daughters Celeste, 3, and Bella, 4.

“This is perhaps the most inhumane and vicious crime I have handled,” Judge Marcelo Kopcow, who has 17 years on the bench, said in the packed and windowless courtroom in Weld County District Court.

The girls’ maternal grandparents, who had urged the prosecution not to seek the death penalty in a deal in which Watts pleaded guilty in November to the murders, spoke in court before the sentence was handed down.

“This is a heartless monster,” said Frank Rzucek, Shanann’s father, sobbing as he stood at a podium just feet away from a seated Watts, who wore an orange prison uniform, his hands cuffed in front of him. “I trusted you to take care of them, not kill them. Then you take them out like trash. You monster.”

An equally emotional statement was made in court by the girls’ paternal grandparents, the parents of the admitted killer.

“I hate what has happened,” said a sobbing Cindy Watts, gripping the podium as her son kept his head down throughout the 45-minute proceeding. “But we will remain faithful as your family. We love you and we forgive you, son.”

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Watts, stoic except for a brief moment when he wiped an apparently tear-streaked cheek on his shoulder, apologized for the crime through his court-appointed lawyer Kathryn Herold. But he declined to speak.

“Mr. Watts asked us to share this... he is sincerely sorry for all of this,” Herold told the judge.

After Shanann Watts and the two girls were reported missing in August from the family’s home in Frederick, about 30 miles (48 km) north of Denver, Christopher Watts went on multiple Denver television stations pleading for their safe return.

The following day, Aug. 15, he was arrested for their murders.

Watts told detectives that he was having an affair. He murdered his victims at home, drove their bodies to an oil field, buried his wife in a shallow grave and dumped the girls in separate oil tanks.