A Colorado father was sentenced to three consecutive lifetimes in prison after a prosecutor detailed for the first time how Christopher Watts planned the August murders of his pregnant wife and two young daughters - apparently in the hope of starting a new life with his girlfriend.

But neither prosecutors nor the surviving relatives of Shanann, Bella and Celeste Watts - each of whom spoke at today's hearing - expected to ever understand how "a seemingly normal person [could] annihilate his entire family," as Weld County District Attorney Michael Rourke put it, and then methodically cover his tracks.

Christopher Watts sits in court for his sentencing hearing at the Weld County Courthouse in Greeley, Colorado. Photo / AP

"You buried my daughter Shanann in a shallow grave and then you put Bella and Celeste in huge containers with crude oil, you heartless monster," Frank Rzucek told the court, as his son-in-law sat behind him clenching and unclenching his jaw, having already pleaded guilty to the murders. "I hope you see that every time you close your eyes at night."

Three months earlier, Watts at first seemed as much a victim as anyone when his wife and daughters - aged 3 and 4 - disappeared August 13.

From his doorstep in Frederick that afternoon, the 33-year-old oil field operator told reporters that he had rushed home from work upon hearing that Shanann's friends couldn't find her or reach her. "In my heart, I believe she is somewhere, and I hope she is safe," he told Fox 31 as police searched the house behind him.

The illusion held up for a day or so, at least on TV. A neighbour down the street recalled how Watts used to play with his girls in a wagon, and his social media feeds were full of love odes to his family and the "little peanut" growing in Shanann's womb, to be named Nico.

But investigators were immediately suspicious after finding Shanann's wallet and keys still in the house.

Christopher Watts sits in court for his sentencing hearing. Photo / AP

A day after the disappearance, a woman named Nichol Kessinger contacted the sheriff's office after seeing Watts on the news, the Denver Post reported. She told investigators that she had been dating him for several weeks, under the impression that he was in the process of finalising a divorce and moving out with his daughters.

"He lied about everything," Kessinger later told the Denver Post.

At today's hearing, the district attorney alleged that Shanann Watts had been trying to save the marriage for weeks - texting her husband and buying self-help books, even as he shopped for jewellery and holiday spots to take his "new love".

"His motive was simple, your honour," Rourke said. "He had a desire for a fresh start."

Police arrested Watts after interviewing Kessinger, and soon announced that he had confessed to killing Shanann in "a rage" - claiming he had walked in on his wife as she was strangling their two girls, too late to save them.

An investigation proved this a lie, too, Rourke said.

"The man seated to my right smothered his daughters," he said, as Watts looked down at his handcuffs, one leg occasionally shaking beneath the table. An autopsy found a cut in Bella's mouth, he said, suggesting that "she fought back for her life" and bit her tongue before she died.

Watts had been watching the girls while Shanann was out of town that weekend, and killed his wife shortly after she returned from the airport early in the morning of August 13.

A bruise in the shape of a finger mark on her neck told investigators that Watts had strangled her slowly and without resistance, Rourke told the court: "The horror she felt as the man she loved wrapped his hands around her throat and choked the life out of her must have been unimaginable."

Besides pleading guilty this month to all three murders and causing the death of his unborn son, Nico, Watts admitted to tampering with their bodies.

Today, Rourke described how surveillance cameras showed Watts going back and forth between the house and his ute in the darkness that morning - loading up the corpses before he drove to a company oil field "to secrete away his family in a place he hoped they'd never be found".

He dug a hole for Shanann and the son she still carried inside her, and stuffed each of his daughters into a separate tank full of crude oil. Bella had to be shoved through the 20cm hatch, Rourke said, leaving a tuft of blonde hair on the side, which investigators in hazmat suits would later discover.

Cynthia and Ronnie Watts openly doubted that their son had really done all these things - even after he pleaded guilty to it all on November 6 in a plea deal that spared him the death penalty.

But today, as the couple addressed the court and their son, they asked only that Watts one day explain himself.

"We love you and we forgive you, son," Cynthia Watts said, turning from the podium to look at him, though he still didn't raise his eyes. "We are forced to question everything. We still don't have all the answers, and I hope one day, Chris, you can help us."

Watts said only two words at the hearing - "no sir," when Judge Marcelo Kopcow asked if he had anything to say.

His in-laws, on the other hand, had much to say to him before he was escorted away to spend the rest of his life in prison.

"Our Shanann loved you with all her heart. Your children loved you to the moon and back," Sandra Rzucek said, still holding her composure.

She lost it as she recalled one of Bella's songs: Daddy You're My Hero.

"I have no idea who gave you the idea you have the right to take their lives," she sobbed. "I didn't want death for you because that's not my right."