A Texas Air Force major has been charged with the murder of his missing wife whose remains were found last week in a rural area in Bexar County.

Bexar County Sheriff’s Office investigators allege Andre McDonald, 40, tried to hide Andreen McDonald’s body with the remains of a dead cow.

“The human remains appeared to have been covered with wood and bones from a nearby deceased cow and set on fire,” according to a Bexar County Sheriff’s Office search warrant affidavit obtained by PEOPLE.

At a news conference Saturday, Bexar County Sheriff Javier Salazar said Andreen’s eight-year-old autistic daughter may have witnessed her mother’s death.

“I am still of the belief this little girl witnessed at least some things in connection with the death of her mom,” he said.

Salazar said they are unsure of the motive.

McDonald, who was charged with first-degree murder Saturday, had been charged in March with tampering with evidence in connection with his wife’s case. At the time of his recent arrest, he was on house detention with a GPS monitor.

During the investigation, investigators discovered that McDonald had allegedly purchased a shovel, an axe, a hatchet, heavy duty large plastic bags, gloves and two five-gallon gas cans on March 2. The items were found inside McDonald’s Porsche Macan which was parked outside the family’s home.

“There were so many things that to us that seemed consistent with someone trying to dispose of a body,” said Salazar.

Detectives also found “clothing that appeared to be worn or used and a claw hammer with brush or woody plant debris” in a trash can and blue jeans in the laundry room that had the blood of both the missing mom and her husband on it, the affidavit states.

In a search of the home, deputies also found a red stain on a light switch in the master bathroom and a burn pile in the backyard that was allegedly used to destroy papers, the affidavit states.

After his wife disappeared, McDonald initially told deputies that his wife was being treated at the Baptist Emergency Hospital but later claimed he didn’t know where she was and that they had an argument the night before she vanished, the affidavit states.

Police found Andreen’s remains six miles away from her home on July 11 when the owner of the 50-acre property near Bulverde asked a friend to remove two cow skulls.

“When he went to where he knew one of the skulls was located he found what appeared to him to be a human skull and bones,” according to the affidavit.

The remains were “co-mingled” with the remains of the cow, the affidavit states.

Her body was also covered with wood and set on fire.

McDonald is being held in Bexar County Jail on a $2 million bond. He has a court appearance scheduled for Sept. 3.

His attorney could not be reached for comment.