(CNN) It became apparent from Chris Watts' research for secluded vacation destinations and jewelry that the married Colorado father had found a new love interest, but what led him to kill his pregnant wife and two young children may never be known, a prosecutor said.

"I don't think he will ever tell us. I don't think he will give an honest assessment of why he did what he did, how he did what he did," Weld County District Attorney Michael Rourke said after Watts was sentenced Monday to five life sentences -- three consecutive and two concurrent -- with no possibility of parole.

Before learning that he would never walk out of prison, Watts sat at the defense table, head down and leg bobbing, as his mother told him she loved him and his slain wife's family called him a monster.

Judge Marcelo Kopcow called the murders in a north Denver suburb "the most inhumane and vicious crime that I have handled out of the thousands of cases that I have seen."

Watts also received an additional 48 years for the unlawful termination of his wife's pregnancy and 36 more years for crimes related to his disposal of the bodies.

The couple's first son would have been named Nico Lee, according to his wife's father.

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The 33-year-old Watts pleaded guilty this month to first-degree murder and other charges in the killings of his wife, Shanann, 34, and their daughters, Bella, 4, and Celeste, 3, before crudely disposing of them at a secluded site where he worked. With approval of his wife's family, prosecutors dropped the possibility of the death penalty.

In court, Shanann Watts' mother, Sandy Rzucek, explained, "I didn't want death for you because that's not my right. Your life is between you and God, and I pray he has mercy for you."

Watts' hands were shackled at his waist and he wore an orange jumpsuit. He rarely looked up during the proceeding -- not even when his mother turned around to face him and tearfully said, "We have loved you from the beginning and we still love you now. ... We forgive you, son."

Two families devastated

Watts' parents, who had previously defended their son and questioned his plea deal during an interview with CNN affiliate KMGH , recanted those remarks and told the court in a prepared statement that the family would not ask for leniency.

"This should never have happened. This is not condonable. This is something that we will never get over," a family representative said, reading a statement as the parents looked on.

Frank Rzucek Sr., Shanann Watts' father, questioned how Chris Watts could have killed people he was charged with protecting before disposing of them like bags of rubbish.

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"I trusted you to take care of them, not kill them," he said. "You have to live with this vision every day of your life, and I hope you see that every day of your life."

While he said he hoped that God had mercy on Chris Watts, he prayed the judge took the opposite tack, he said.

Frank Rzucek Jr., Shanann Watts' brother, had Rourke read his statement, in which he said he can't sleep and is anguished to know Chris Watts robbed him of the privilege of being an uncle. Rzucek Jr. wrote his statement from a motel room where his parents have cried themselves to sleep, the prosecutor said.

"There isn't a day that goes by that I don't cry for my family," Rourke said, reading Frank Jr.'s statement. "You don't deserve to be called a man. What kind of person slaughters the people they love the most?"

Gruesome disposal of bodies

Before Watts was sentenced, Rourke revealed horrific details of how Watts strangled his wife and smothered their daughters.

Watts strangled his wife with his bare hands, not in an uncontrolled rage as he had claimed, but deliberately and viciously for at least 2 minutes. He then suffocated Bella and Celeste. While Bella fought back, Celeste had no external injuries, the prosecutor said.

He then backed his vehicle in to the driveway, loaded up the bodies in three trips and drove them to a work site, a place he suspected they'd never be found. He buried his wife in a shallow grave before stuffing his daughters into tanks, Rourke said.

"Imagine this: This defendant took those little girls, put them through a hatch 8 inches in diameter," the prosecutor said. "Bella had scratches on her left buttocks from being shoved through the hole."

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When his coworkers arrived at the site later that morning, Rourke said, they described his behavior as perfectly normal. Millions across the country were riveted and local law enforcement searched for the woman and girls, while Watts was texting his new girlfriend about their future, he said.

His sentencing came days after a woman he had been dating told The Denver Post that he lied to her about being near the end of divorce proceedings. She cooperated with police before his arrest, she told the newspaper.

"You don't annihilate your family and throw them away like garbage," Rourke told the court. "Why did they have to lose their lives in order for him to get what he wanted?"

An attorney for Chris Watts told Judge Kopcow her client is "devastated," and though Watts realizes his words are hollow, he is "sincerely sorry for all of this."

The only time Watts spoke during the proceeding was when Kopcow asked him if he wished to deliver a statement. "No, sir," the former father replied.

Toxicology results showed that Shanann had a blood-alcohol content of .128, which is consistent with decomposition, Rourke said. It does not mean she consumed alcohol or was drunk, he said.

Several substances found in the young girls' bodies can be attributed to the crude oil and water in which they were submerged, Rourke said.

Late wife noticed suspicious behavior, prosecutor said

Rourke told reporters that Watts had visited museums, bars and restaurants with his new love interest. Other evidence of that relationship included photographs of the pair at the Great Sand Dunes National Park & Preserve in southern Colorado while Watts' wife was in North Carolina, Rourke said.

Watts had also researched secluded vacation spots in Aspen where he had planned to take the new girlfriend, Rourke said, referring to Watts' Internet searches while his wife and daughters were away.

"For whatever reason, in his mind divorce wasn't an option," the prosecutor said.

"He was doing all that kind of research that you would think that somebody in a brand new relationship would be doing, but he was still married with two daughters and a son on the way," Rourke said.

Shanann Watts noticed her husband's suspicious behavior, Rourke said.

"We knew that she was getting alerts on her phone of credit cards being used at a restaurant in north Denver at a time that she was in North Carolina for amounts of money that one person couldn't possibly probably consume reasonably," the prosecutor said.

"We know that she confronted him about that numerous times," Rourke said. "We knew that he was never completely forthcoming with her."

A plea from the front porch

When Shanann Watts and the two girls went missing in mid-August, Chris Watts first told reporters they had gone to a friend's house before later pleading for his family's return in an interview with CNN affiliate KMGH

"My kids are my life," Watts said, standing on the front porch of their home in Frederick, about 30 miles north of Denver. "Those smiles light up my life."

Watts told the station his wife came home from the airport about 2 a.m., and he left for work about 5:15 a.m. He said the two had an emotional but civil conversation before he left, but he didn't elaborate.

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Separately, he told law enforcement that on the morning of August 13, before leaving for work, he had informed his wife that he wanted a separation. They had not argued, but they were both upset and crying, he told police, according to an affidavit.

Chris Watts claimed he had called and texted his wife about three times that morning with no reply, he told KMGH. He said he realized something was wrong when Shanann Watts' friend arrived at the couple's home around noon.

"She just got back from Arizona, and I figured she was just busy, but when her friend showed up, that's when it just registered -- like all right, this isn't right," Watts said.

Story didn't add up

Three days later, the body of Shanann Watts,15 weeks pregnant, was found in a shallow grave. The bodies of Bella and Celeste were found in commercial oil tanks at a company where Watts had worked.

Chris Watts' mugshot

Rourke said Watts became a suspect "very, very early on."

"Many of the things that he was saying just weren't quite adding up," Rourke said Monday.

One key piece of evidence was sheets from the bed in the Watts' master bedroom that were found at the crime scene, prosecutors said.

After authorities found the bodies, Chris Watts changed his story, telling police he had strangled Shanann Watts in a fit of rage after seeing his wife choking Celeste.

On November 6, Watts pleaded guilty to nine charges, including three counts of first-degree murder and unlawful termination of a pregnancy.

Other woman tells her side of story

According to an unsealed Weld County arrest affidavit , investigators say they discovered Watts was "actively involved" in an affair with a co-worker.

It's not clear if that was Nichol Kessinger, 30, who told The Denver Post she met him while working for the environmental department of a petroleum contractor.

"We had just met," Kessinger told the newspaper. "I barely knew him."

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He wasn't wearing a wedding ring when they were introduced, and she thought he was attractive, soft-spoken and a good listener, she said. When they first met outside of work in late June, he told her he was in the final stages of a divorce. She never met any of his relatives or friends, Kessinger told The Post.

Watts sent her a text August 13 to say his family had disappeared, and she was confused about why the media was at his house.

"When I read the news, I found out he was still married and his wife was 15 weeks pregnant," Kessinger told the newspaper.

She peppered him with questions via phone calls and text, she said, and he changed his story about the divorce, showed little emotion about his family's disappearance, and tried to change the subject.

"It got to a point that he was telling me so many lies that I eventually told him that I did not want to speak to him again until his family was found," she told the paper, explaining that she called police on August 15 to report Watts' lies.

Rourke said she "was absolutely instrumental" in the investigation.