Two young Colorado girls who, with their mother, were killed this past week may have been strangled before their bodies were dumped in an oil well, according to a court document.

A motion filed by lawyers defending Christopher Watts, an oil and gas worker, who investigators believe killed his pregnant wife and daughters, asks that DNA samples be taken from the children's necks.

The document cites an expert who argued that DNA would still be present on the bodies - even though they had been submerged in crude oil for four days before they were found - but that evidence would be lost once the autopsies were performed.

A Weld County judge, however, denied the request, the Denver Post reported. And authorities said that they had finished the autopsies on the victims, though they did not say how they died.

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The document offers the first glimpse into what may have happened to Shanann Watts, 34, and her two daughters when they disappeared last Tuesday.

The following day, Christopher Watts stood in front of a parade of news cameras and told reporters that his children were his life. His family had just vanished, he told Denver 7 ABC. "I'm living in a nightmare, and I can't get out of it," he told NBC affiliate KUSA.

But the media interviews were over on Thursday, when police vehicles showed up at the Watts house. Reporters photographed officers removing bags of evidence and towing away a ute. The Frederick Police Department arrested Christopher Watts, 33, on suspicion of first-degree murder and evidence tampering - three counts each.

Watts was fired the same day he was arrested, the company said.

By Friday, authorities said that they had found the bodies of 4-year-old Belle, 3-year-old Celeste and their mother, all "in close proximity" to each other on property owned by Watts' employer Anadarko Petroleum, one of the state's largest oil and gas drillers.

Christopher Watts' lawyers sought to preserve DNA evidence they said would still be present on the bodies, saying that once autopsies are finished, the evidence will be "lost forever," according to the motion.

The lawyers said they consulted DNA expert Richard Eikelenboom who said in court records that he has experience taking samples from dead bodies and "getting good results after strangulation".

"This DNA can be retrieved with a double swab technique. DNA scientists are familiar with this technique and an experienced person should take this samples. In my opinion the presence of oil will not destroy the DNA," Eikelenboom said.

Eikelenboom added that the hands of the children, as well as the hands and nails of their mother should all be sampled as well. He and defence lawyers did not seek swabs from the mother's neck.

Eikelenboom is a forensic scientist who specialises in DNA trace recovery and has examined "hundreds of crime scenes," according to Independent Forensic Services, a company he owns with his wife.

In 2016, a Denver district court judge rejected Eikelenboom as a DNA expert after he admitted to a prosecutor that he had no direct DNA extraction or analysis experience, that he operated an unaccredited lab, that he failed basic proficiency tests in 2011 and 2012, and that he was "self-trained" in running DNA profiles, the Denver district attorney's office said. The agency also said that Eikelenboom "has committed fundamental DNA analysis errors by not following accepted scientific standards in the DNA field." Eikelenboom, who's from the Netherlands, said the district attorney's office misrepresented his experience. He didn't have experience with DNA extraction and analysis back in the 1990s, before advancements in DNA technology, he said, "but that doesn't mean I didn't do it in 2005 or in 2016, or that I'm not fully accredited."

A photograph sits amid the tributes as they grow outside the victims' home. Photo / AP
A photograph sits amid the tributes as they grow outside the victims' home. Photo / AP

He added: "It's just manipulation of facts,"

Eikelenboom's wife started Independent Forensic Services in Colorado in 2003, according to its website. He joined the company in 2005. Independent Forensic Services, headquartered in the Netherlands, is accredited by the Dutch Accreditation Council and by the American Society of Crime Laboratory Directors.

How the DNA evidence would have fit into Christopher Watts' defence is unknown.

Investigators have said little else about the deaths. The district attorney has until tomorrow to bring formal charges, at which point the arrest affidavit may be unsealed, revealing how Watts became a suspect.

The family's life seemed idyllic. They lived in a big two-storey house and Shanann Watts was pregnant with the couple's third child.

Just two months ago, she sent her husband a picture of her first ultrasound. "Little peanut!" Christopher Watts replied.

"I love Chris! He's the best dad us girls could ask for," Shanann Watts wrote in a Facebook post sharing the text exchange with her husband.

Their life, apparently, wasn't perfect. The family remained caught between a promising future and financial strain from debt and other obligations.

n 2015, two years after buying their house, the Wattses filed for bankruptcy.

They were drowning in a US$400,000 mortgage and more than US$50,000 in credit card debt, bankruptcy court documents show. His income as an operator for Anadarko Petroleum and hers from a children's hospital weren't nearly enough.

They struck a deal with their creditors, and Shanann Watts got a new sales job doing direct sales for a "lifestyle system" called Thrive.

They stayed in their big two-storey house, and the family kept growing.

In a statement to CBS4, Shanann Watts' family said her husband confessed to the killings.

"It is with deep hurt, confusion and anger to confirm our beautiful cousin Shanann Watts, her unborn child, and her two angelic daughters, Bella (4), Celeste (3) were viciously murdered by husband Chris Watts," the family said.

- additional reporting AP