Volunteers scour the desert for Nancy Guthrie

The disappearance of US Today show host Savannah Guthrie's mother three weeks ago has inspired a small number of volunteers to launch their own searches in the dense desert near her home in hopes of cracking the case.

The Pima County Sheriff's Department said while it appreciates the concern for Nancy Guthrie, it asked people inquiring about volunteering to give investigators space to do their jobs.

"We all want to find Nancy, but this work is best left to professionals," the agency said in a statement over the weekend.

Nancy Guthrie, 84, was last seen at her home just outside Tucson on January 31 and was reported missing the following day.

Authorities believe she was kidnapped, abducted or otherwise taken against her will. Drops of her blood were found on the front porch, but authorities haven't publicly revealed much evidence.

Despite the sheriff's request for people not to search on their own, volunteers have continued to look.

A small group reported finding a black backpack on Sunday, but it wasn't the same brand as one identified in video surveillance that the FBI released of a masked man at Guthrie's home the night she disappeared.

A sheriffs' spokesperson told Tucson television station KOLD that the bag and its contents didn't appear to be viable leads.

Two women from the group Madres Buscadoras de Sonora, or "Searching Mothers of Sonora," who were carrying digging tools Sunday outside of Guthrie's home, said they, too, would join the search.

They posted fliers on Guthrie's mailbox with her picture and their contact information.

Tony Estrada, the former long-time sheriff in neighbouring Santa Cruz County, said volunteer searchers had good intentions in wanting to help and can serve as a force multiplier, but it was crucial their efforts be co-ordinated with law enforcement.

"You can't have people all over the place looking for something and not reporting to anybody or letting them know that they're going to be in that area," Estrada said.

"They may be trampling into things that may come out to be helpful in the future."