Authorities expected to release affidavit explaining why they think Chris Watts killed his family

FREDERICK, Colorado -- On Monday, authorities in Colorado are expected to release an affidavit explaining why investigators believe 33-year-old Christopher Watts killed his pregnant wife and two young daughters.

Christopher faces one count of first-degree murder after deliberation, two counts of first-degree murder - position of trust, and three counts of tampering with a deceased human body, after the bodies of his wife, 34-year-old Shanann Watts, and two daughters, 3-year-old Celeste and 4-year-old Bella, were found on the property of Anadarko Petroleum, one of Colorado's largest oil and gas drillers.

Records show Christopher was employed by the company in 2015.

Officials said the bodies of the young girls were found inside oil and gas tanks that "were mostly full." Investigators believe Christopher placed the girls in there to conceal the smell from passersby.

Shanann was reported missing on Aug. 13 by her friend Nickole Atkinson after she missed a doctor's appointment.

Atkinson said she felt something was off and called police.

Atkison spoke exclusively with Good Morning America (GMA), saying things weren't as perfect as they seemed.

RELATED: Friend who says she was the last to see Shanann alive knew something was 'seriously wrong'
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Nickole Atkinson believes she was the last person to see her friend alive, before she and her two daughters were found dead three days later.

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