Authorities are investigating alleged ransom notes in the disappearance of US news anchor Savannah Guthrie's mother, the sheriff's office said.

The Pima County Sheriff's Department said it was "aware of reports circulating about possible ransom note(s)" and were "taking all tips and leads very seriously".

The BBC's US partner, CBS News, reported that a note was sent to KOLD-TV, its affiliate in Arizona. The sheriff's department did not confirm if the notes were authentic.

Nancy Guthrie, 84, was last seen near her home near Tucson, Arizona, on Saturday. Investigators said they believe she was taken against her will.

Pima County Sheriff Chris Nanos said the FBI had reviewed the note and shared it with Savannah Guthrie, who is an NBC presenter.

"It's like any piece of evidence," Nanos told CBS. "You give it to us, you give us a lead, we're going to look at every aspect of that lead."

When asked at a news conference earlier on Tuesday if investigators were searching for someone still alive, given that Nancy Guthrie has been without needed medication for days, Nanos said: "We hope we are."

He did not provide a possible motive and also said investigators had not been able to access footage from the home's surveillance cameras.

Nanos told the briefing his department was working with technology companies to try to view footage from Saturday night, after family members dropped off Nancy Guthrie at home, around 21:30 local time.

Members of her church called her family when she did not attend Sunday services, around 11:00 the next day.

When authorities arrived at Nancy Guthrie's property, the scene there caused "grave concern", Nanos has said.

While there was no initial indication she could have been targeted because of her name, the sheriff has said "we can't dismiss that".

"We do believe that Nancy was taken from her home against her will. And that's where we're at," Nanos told reporters on Tuesday.

He asked the public to call in with leads or tips, saying investigators would follow through on each one.

Police found blood in Nancy Guthrie's home and are testing it to determine whose it is, CBS reported.

Nanos said: "I'm not saying saying there's blood inside that house or outside that house."

Physical evidence samples, such as fingerprints or DNA, have been submitted for analysis, but so far none of the lab's findings "would tell us 'this is who did this'", Nanos said.

Customs and Border Protection are providing tracking dogs, said Nanos.

On Monday evening, Savannah Guthrie said on social media: "We believe in prayer. we believe in voices raised in unison, in love, in hope. we believe in goodness. we believe in humanity. above all, we believe in Him.

"Thank you for lifting your prayers with ours for our beloved mom, our dearest Nancy, a woman of deep conviction, a good and faithful servant."

Nancy Guthrie has been described as "not in good physical health", but has no reported cognitive issues, according to Nanos.

In an earlier news conference, the sheriff said homicide detectives were called to the scene, which he said was not standard protocol in a missing person case.

Savannah Guthrie, who hosts the Today show, has not been on air this week.

She had been due to co-host the Winter Olympics opening ceremony for NBC on Friday, but will no longer take part in coverage to focus on being with her family, a spokesperson for NBC said.

"Our hearts are with her and the entire Guthrie family as the search continues for their mother," NBC said.

On Tuesday, US President Donald Trump said he planned to call Savannah Guthrie later and that he would be willing to commit more federal agents to help with the investigation.

"I think it's a terrible thing," he said at a White House event.

"I always get along very good with Savannah," he added. "Very unusual situation." (BBC)

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