The search for a missing 4-year-old Oklahoma girl is now being considered a “recovery operation,” the Oklahoma State Bureau of Investigation said Monday.

“Special Agents from the Oklahoma State Bureau of Investigation, along with our partners at the Oklahoma Highway Patrol (OHP), have been searching areas of Caddo County for the toddler’s remains,” OSBI said in a statement.

Athena Brownfield was reported missing last Tuesday after her 5-year-old sister was found unsupervised by a postal carrier at about 2 p.m. local time outside the Cyril, Oklahoma, home of couple Ivon and Alysia Adams. Authorities said the two children had been in the Adams’ care for “at least a year.” Both Ivon and Alysia Adams have been arrested in connection with Brownfield’s disappearance.

Ivon Adams, 36, was taken into custody Thursday in Arizona and is awaiting extradition to Oklahoma, OSBI said. He faces one count of first degree murder and one count of child neglect, OSBI said Monday. 

Alysia Adams, 31, was also taken into custody Thursday and is facing two counts of child neglect, OSBI said. She is being held at Caddo County Jail, OSBI said.

Brownfield and her sister are related to Alysia Adams, according to OSBI. The girls’ biological parents have been interviewed by authorities and are cooperating with the investigation, OSBI said. The 5-year-old sister has been placed in protective custody, OSBI said Saturday.

OSBI on Monday asked people not to go searching for Brownfield on their own, and warned that those who do say could be trespassing and run the risk of “interfering with the investigation by contaminating potential evidence.”

Brownfield’s disappearance sparked a huge search effort last week, which included the Oklahoma Highway Patrol’s emergency response team and local volunteers conducting a grid search of the entire town. That search included an examination of every known vacant house in the area and of local waterways, OSBI said.