CYRIL, Oklahoma (AP) — An effort to locate a missing 4-year-old Oklahoma girl who authorities believe was killed by a caretaker has become a search for her remains, officials said Monday.

The search for Athena Brownfield, missing since last week, is now considered a “recovery operation,” the Oklahoma State Bureau of Investigation said in a news release.

Authorities determined Athena was missing after a postal carrier found her 5-year-old sister wandering alone on Jan. 10 outside of the home of the couple who had been caring for the sisters in Cyril, a city of around 800 people located about 70 miles (110 kilometers) southwest of Oklahoma City.

Ivon Adams, 36, was arrested Thursday in Phoenix and is awaiting extradition to Oklahoma. He faces charges of first-degree murder and child neglect, according to the OSBI.

Alysia Adams, 31, was arrested in Oklahoma and faces two counts of child neglect. The OSBI said she is related to the sisters.

Jail records do not list attorneys for either who could comment on the accusations against them.

Areas of Caddo County, where Cyril is located, are being searched for Athena’s remains, the OSBI said Monday. The agency asked the public not to search for Athena’s remains, saying that people could end up trespassing or contaminating potential evidence.

The sisters’ biological parents have been interviewed by authorities and are cooperating with the investigation, the OSBI said. Athena’s sister was taken into protective custody.