A monster dad strangled his two young children before hiding their bodies along with that of their pregnant mother in oil tanks, new court papers suggest on the eve of what would have been the mother's party to reveal her baby's gender.

Lawyers for Christopher Lee Watts, 33, filed a motion on Friday in Weld County, Colorado, asking for DNA to be taken from the necks of the little girls’ bodies, and it strongly suggested they were strangled.

There has been no official cause of death announced in the case, after the bodies of Shanann Watts, 34, and the couple's two daughters, four-year-old Bella and three-year-old Celeste, were recovered on Thursday from the oil field where Christopher worked.

Christopher, 33, is being held without bail and faces three counts of first-degree murder and three counts of evidence tampering.

Formal charges are expected to be filed on Monday, and prosecutors are believed to be weighing the death penalty - a rarity in Colorado, which has executed only one offender since capital punishment was reinstated in 1977.

Christopher Lee Watts, 33, faces charges in the murders of his wife Shanann and their daughters, four-year-old Bella and three-year-old Celeste. The family is seen above

A heartbreaking new court filing suggests that Celeste and Bella (above) were killed by strangulation, as it requests DNA swabs to be taken from the girls' necks

Watts is seen in court on Thursday. He is being held without bail, and prosecutors are believed to be weighing whether to pursue the death penalty in the case - a rarity in Colorado

Christopher Watts' public defender filed this motion on Friday morning, indicating autopsies for the girls were imminent and requesting DNA swabs from their necks

The new filing by court-appointed defense lawyer Megan Ring asks the judge to allow a DNA expert to be called to examine the bodies of the two young girls, and noted that autopsies were scheduled to be performed on Friday.

The lawyer said she wants to make sure DNA from the little girls’ bodies is maintained and she has not been able to ascertain whether the prosecution is taking steps to do so.

‘It does not seem clear that the coroner, pathologist, or anyone working on behalf of the prosecution team in autopsy plan to take swabs of the necks of the two decedent children,’ she said in a two-page filing to a court in Weld County, Colorado.

She specifically asked for ‘several swabs of the neck’ to be taken, along with control swabs on other parts of the girls’ bodies. She said without a defense expert taking the swabs that evidence could be ‘lost forever.’

Ring asked that DNA expert Richard Eikelenboom be allowed to do the testing, saying that he believed that DNA would remain on the necks of the girls ‘in spite of the fact that the remains were in oil for four days’ - revealing for the first time that the bodies were hidden on Monday.

Christopher worked for oil drilling company Anadarko, while Shanann sold weight-loss patches for the multi-level marketing firm Le-Vel

Christopher is seen mowing the grass behind the couple's suburban home in Frederick

The filing does not suggest taking any swabs from the neck of Shanann, and sheds no light on the pregnant mother's cause of death.

The gruesome revelation of the girls' likely cause of death comes as Shanann's friends lament on what would have been the eve of the gender reveal party for her unborn baby.

Shanann's brother Frankie Rzucek said in a heartbreaking Facebook post on Thursday that the unborn child was a boy, and the couple planned to name him Niko.

Colorado is one of 12 states that does not bring murder charges for the homicide of an unborn child, and Christopher faces only three murder counts.

Ashley Bell, a tanning salon owner who became close friends with Shanann after she began coming in as a customer, told KDVR that the baby's gender reveal party had been planned for Saturday in the Watts' suburban home in Frederick, on the northern outskirts of Denver.

Instead, the home has become a somber memorial, with a small mountain of stuffed animals piling up outside.

On Friday, even people who did not know the couple were passing the home and leaving teddy bears, flowers and candles in the front yard.

Shanann is seen showing off her baby bump. She had planned to have a party announcing the gender of her third child, a boy that the couple had decided to name Niko

The couple's home, where police say the murders were committed, is seen on Friday

Instead of a gender reveal party, the home is now a somber site of mourning, with even strangers visiting to pay tribute to the slain mother and children

Police say that the children and their mother were killed inside the home before their bodies were hidden in the oil field.

Investigators have not said when the murders were committed, but the new court filing indicated the bodies were hidden on Monday.

Shanann was on a work trip to Arizona from August 10 to August 12. She sold weight-loss patches for a multi-level marketing firm, while Christopher was an operator for oil drilling company Anadarko.

Christopher claimed in a televised interview that Shanann returned home between 1am and 2am on Monday August 13, and that they stayed up having an 'emotional conversation' until he left for work at 5.15am.

He continued to maintain his innocence for several days, making public appeals for his family's safe return, until police said he confessed to the murders on Wednesday.

Investigators have not revealed a motive in the case. While some friends of the couple have said that the relationship was under strain and there may have been infidelity, others said the marriage was strong and there were no signs of trouble.

The family is seen leaving Colorado for a trip to North Carolina in late June. Shanann and the girls stayed for six weeks visiting her family, while Christopher returned after one week

Shanann and the girls had recently returned to Colorado on August 7, after spending six weeks with her family in North Carolina. Christopher returned to Colorado after the first week of the trip as previously planned, according to social media posts.

The couple declared bankruptcy in 2015, and despite Shanann's frequent Facebook posts boasting of the Lexus and frequent paid trips she was awarded for selling health supplements, the couple was facing a $1,500 civil suit from their homeowners' association.

They purchased their five-bed, four-bath home for $400,000 in 2013, and their mortgage payment was about $3,000 a month, according to bankruptcy records.

A candlelight vigil for the slain mom and girls was held outside of the home on Friday evening.

Christopher Watts is next due in court on Tuesday. Prosecutors may take several weeks to consider whether to pursue a death penalty case against him.

At a minimum, he will receive life in prison without the possibility of parole if convicted.