The sheriff investigating the kidnapping of Nancy Guthrie, mother of US TV presenter Savannah Guthrie, says authorities believe they have a motive, but have not identified a suspect.

Pima County Sheriff Chris Nanos told NBC News that "we believe we know why he did this". He said authorities "believe it was targeted", but they were "not 100% sure of that".

Guthrie, 84, has been missing since she was taken from her home in Tucson, Arizona, on 1 February.

A nationwide search has unveiled certain clues, including DNA evidence and ransom notes, but no suspect has been identified.

Nanos declined to elaborate on the suspected motive, as he said it could compromise the integrity of the ongoing investigation to do so.

The sheriff did warn there was "absolutely" a chance that the unknown suspect who kidnapped Guthrie 41 days ago could strike again.

"It'd be silly to tell people, 'Yeah, don't worry about it. You're not his target,'" Nanos said in Thursday's NBC News interview. "Don't think for a minute that because it happened to the Guthrie family, you're safe."

Nanos warned residents in Guthrie's affluent Catalina Foothills neighbourhood to "keep your wits about you".

Savannah Guthrie has posted multiple videos on social media making direct appeals to her mother's captor, and is offering $1m (ยฃ760,000) for information that leads to her return.

"She may be lost, she may already be gone," Savannah said in a video shared in February.

There have been arrests and detainments in the case, but they have not led officials to find the actual suspect.

A California man arrested on suspicion of sending fake ransom notes is facing federal charges, but does not seem to be involved in Nancy's disappearance.

A delivery man was also detained for questioning, but he was released shortly after he was cleared by authorities.

So far, the lone glimpse officials have of the suspect is of a masked man whose image was captured on the doorbell camera outside Nancy Guthrie's home.

The disappearance of the mother of TV presenter Savannah Guthrie captivated the US, but police have not made any arrests.

The Today programme host visited her colleagues in the office on Thursday, nearly five weeks since her mother disappeared.

It has been almost a month since 84-year-old Nancy Guthrie disappeared in the middle of night from her home in Arizona.

Fighting back tears, the missing woman's NBC host daughter says they accept she may be dead.

A masked man in doorbell camera footage is thought to have visited Guthrie's home before the night she disappeared.