Two ransom notes sent in relation to Nancy Guthrie's disappearance have been made public in the hope someone will recognise the handwriting.

The 84-year-old mother of Today show host Savannah Guthrie has been missing since February and now the Pima County Sheriff's Department (PCSD) made a fresh appeal for information, using the messages which were sent to Nancy's family via different news outlets.

A press release from the department read: "These communications reveal distinctive patterns of word choice, syntax, and phraseology that reflect the writer's unique linguistic style.

"They also provide valuable insight into the person's mindset, motives and the evolution of their tone over time."

Authorities believe that the' "distinctive linguistic characteristics" in the letters may be "recognisable to someone who knows or has interacted with the notes' writer. A friend, family member, coworker, classmate, or acquaintance may recognise these patterns of expression and possess information that could help identify the individual responsible."

The first note was sent to KOLD-TV 13 News, a local CBS affiliate on 2 February, the day after Nancy was reported missing, and addressed to Savannah.

It read: "We have your mother, Nancy. She is safe but scared.

"She will be held for ransom, and once payment is received, she will be released unharmed. We will be holding her for a maximum of 7 days."

The broadcaster was asked to send "Bitcoin in the amount of $4 million USD before 5 p.m. on Thursday."

The message added: "If payment is not received, the ransom will be increased to $6 million USD, which will need to be paid by 5 p.m. on Monday.

"Once payment is received to the Bitcoin address below, she will be released within 12 hours of deposit to a safe drop-off location back in Tucson."

And the alleged kidnappers warned Savannah her mother would be "killed" of the payment wasn't received by the second date.

They wrote: "Your mother is aware of this, and her life is in your hands. It is in the best interest of everyone to have this completed as soon as possible. You will not be able to contact me from here on out, there will be no negotiation. Do not play games, law enforcement will not be able to help you."

The second note, addressed to the Guthrie Family, was sent to KOLD on 6 February and claimed Nancy had died.

It read: "We did not fully grasp the seriousness of her physical condition.

"We never intended to hurt her, that was not our intention. She perished shortly after she was taken. We believe it was heart-related.

"She is buried in nature now. Nothing you could have done could have changed the outcome. We want your family to know this and hope you all can find peace. We are truly sorry."

PCSD also noted they and the FBI have recovered videos from Nancy's home in Tucson, Arizona, which feature an unidentified male who "knows what happened" to the matriarch and appealed for "continued assistance" from the public.

They added: "Two videos recovered from Nancy's doorbell camera may be from two separate days.

"These videos are critical to our understanding of the unidentified male's behaviors and choices. They indicate he took steps to prepare for the event on the night of January 31 to February 1.

"We know there are individuals familiar with this unidentified male who may have observed changes in his behavior around the timeframe of the videos."

They urged people to look out for "a change in mood or displays of anxiety, nervousness or irritability, a change of physical appearance: hair colour or cut, growth or removal of facial hair, etc., unexplained injuries (cuts on hands, bruises, etc.) during the period Nancy was last seen, normal routine changes (missing work, classes or appointments), sudden departures from the area around the time of her disappearance, vehicle cleaning, or repairs, unexplained interest or disinterest in Nancy Guthrie's neighborhood and noticeable intense interest or disinterest in the investigation or an unexplained knowledge of the situation."

Authorities stressed it is never "too late" to come forward with information.

The statement concluded: "In some cases, there are times when people who may have knowledge initially do not come forward. This may be due to their close relationships with those who might be involved as well as a concern for their own safety, reputation, or standing in the community and amongst their friends.

"We recognise relationships and loyalties change over time, as do people and their perspectives. It is not too late to come forward. Regardless of how insignificant you think your information may be, or whether you think we are already aware of it, please contact us."