After she was allegedly killed by husband Chris Watts, Colorado mom Shanann Watts was buried in a "shallow grave" on property belonging to Chris' employer

After she was allegedly murdered last week by her husband, Chris Watts, Colorado mom Shanann Watts was buried in a “shallow grave” on property belonging to Chris’ then-employer, according to multiple news outlets.

Citing court documents filed by prosecutors, ABC News, the Denver Post and local TV station KUSA report that Shanann’s remains were “recovered from a shallow grave near an oil tank.”

Authorities previously announced that Shanann, then 15 weeks pregnant, had been found dead on Thursday on the property of Anadarko Petroleum, where Chris reportedly worked as an operator until being fired on Wednesday — the same day as his arrest.

The bodies of the Watts’ young daughters, 4-year-old Bella and 3-year-old Celeste, were also found on Thursday not far from Shannan, according to investigators.

Causes of death have not been released, though a motion filed by Chris’ attorney raised the possibility — in requesting that DNA evidence be recovered from their necks — that at least Bella and Celeste were strangled.

The prosecutor’s filing revealing Shanann’s burial location was in response to defense motions. It showed that the girls’ bodies had been concealed “inside oil tanks near” Shanann’s grave.

According to KUSA, the defense stated in a previous motion that Bella and Celeste had been in oil for four days — essentially the length of time that they and their mom were missing. The disappearances were reported Monday afternoon some 12 hours after Shanann returned home, to Frederick, from a business trip.

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Shanann Watts’ daughters
| Credit: Frederick Police

Chris was arrested Wednesday night and remains in custody, accused of first-degree murder and tampering with a body in all three deaths. He will return to court this week to face formal charges.

His attorney did not respond to PEOPLE’s request for comment.

One of the defense’s motion, requesting DNA be gathered from around the girls’ necks, was denied on Friday with the judge explaining that he would not dictate work for the medical examiner, according to The Coloradoan.

Autopsies were conducted Friday on Shanann and her daughters, the paper reports. A motive has not been confirmed.

“They were the couple that you’d look at and then turn to your husband and be like, ‘Why aren’t we like that?’ ” friend Kris Landon tells PEOPLE. “They seemed so supportive of each other, it was their public face. I had no idea what was going on behind closed doors, and I don’t think many people did.”

• With reporting by STEVE HELLING