Police on Monday were digging in the yard of the family of a missing 6-year-old girl whom investigators believe was the victim of a homicide as long as 18 months ago.

Police spokesman Marcus Dudley would not say what prompted officers to begin digging or what other work was going on at the house, saying it was a criminal investigation.

Meanwhile a family spokesman said seven children who lived in the same home as missing Aarone Thompson will remain in protective custody for at least a week.

Investigators say Aarone's father and his live-in girlfriend are "persons of interest" in the case, but no arrests had been made and no body had been found.

Aaron Thompson reported his daughter missing on Nov. 14, saying she left the house after he would not let her have a cookie. Police have said Thompson is lying, but they did not elaborate.

The lawyer representing Thompson and his girlfriend, Shely Lowe, did not immediately return a telephone message Monday.

The seven other children living with Thompson and Lowe were taken into protective custody and interviewed by investigators last week. A juvenile court held a hearing the status of the children Monday but left them in protective custody, scheduling another hearing for Nov. 29, family spokesman Sam Riddle said.

Riddle said Thompson's mother is seeking custody of one of the seven, 11-year-old Aaron, her grandson.

Over the weekend, members of NecroSearch, a nonprofit group that specializes in finding hidden graves, joined the investigation into Aarone's disappearance.

Volunteers from the Fort Collins-based group searched the property where Aarone lived two days after cadaver dogs searched inside and outside the house.

Family members have questioned the police decision to give up on finding Aarone alive and instead focus on the homicide investigation.

Investigators said last week that Thompson and Lowe had turned down their requests for interviews.