The FBI assessment of inauthenticity pertains to the two ransom notes reported in early February, days after 84-year-old Nancy Guthrie vanished, and a third, more recent ‌message from someone claiming to know the kidnappers' identities, the official said.

"None of the ransom notes are believed to be genuine," the FBI official told Reuters, speaking on condition of anonymity to discuss details of an active investigation.

A second law enforcement source familiar with the matter confirmed the FBI assessment of the ransom notes.

The disclosure that the FBI has discounted the veracity of the three notes - two of which were widely reported to have been communications from kidnappers - seemed to raise doubts ​about investigators' fundamental premise that Nancy Guthrie was abducted for ransom to begin with.

A spokesperson for the Pima County Sheriff's Department, which is leading the overall investigation, declined to comment, citing its ​agreement to refer all inquiries regarding ransom notes to the FBI.

"We don't have any updates, other than this is still an active investigation," the sheriff's spokesperson, ⁠Angelica Carrillo, said. She added that DNA samples and video evidence collected in the case "remain under forensic analysis."

All three messages in question were initially delivered to various media outlets, including celebrity news site TMZ.com, before ​they were turned over to authorities for review.

Savannah Guthrie, 54, longtime co-anchor of the NBC morning news program "Today," has referenced ransom demands in video messages she and her siblings have posted on social media, urging kidnappers ​to open a direct line of communication with her family and pleading for her mother's return, saying in one video, "we will pay."

CRYPTOCURRENCY PAYMENT WENT UNCLAIMED

The elder Guthrie, who had been in frail health with limited mobility, was last seen alive at her home in Tucson, Arizona, on January 31, after spending an evening with her older daughter, Annie Guthrie, and son-in-law.

The FBI, which has taken the lead in examining any purported ransom notes or other communications from suspects in the case, ​had previously declined to say publicly whether any of the messages under review were considered credible.

Investigators determined that the first two originated from the same sender, the official told Reuters, though how that conclusion was ​drawn was not specified.

The first note, according to TMZ, demanded a sum "in the millions" to be paid in cryptocurrency and set two deadlines for payment - on February 5 and February 9. The second note was reported by NBC News ‌last week to ⁠have referred to Guthrie at that point as having died, without making an apology or demanding any payment for the return of her body.

In a bid to test the authenticity of the first note and possibly trace the ransom demands back to the perpetrators, the FBI early on deposited a small amount of cryptocurrency in an account as instructed in the message, but the money was left untouched in the account and never taken, the official said.

It was on that basis and by other unspecified means that the FBI reached the conclusion that the two ransom notes that it determined to be of a common origin were nevertheless ​sent by a person or persons not actually connected ​with Guthrie's disappearance, according to the official.

The ⁠FBI has additionally discounted the authenticity of a third note that TMZ reported it received last week from someone claiming to know the identities of Nancy Guthrie's abductors and to have video of the "main guy" involved in her kidnapping, as well as of the victim on the day she died.

The FBI official did not ​reveal how investigators ruled out the third note as fake.

Reacting to last week's ransom note headlines from NBC News, the home network of "Today," Savannah Guthrie ​took time on her show ⁠to appeal to the public again for answers to the fate of her mother, urging anyone who might know something to come forward. Guthrie also reminded viewers of the $1 million reward being offered by her family, whom she described as being in "agony" since her mother vanished.

Authorities confirmed soon after her disappearance that DNA tests showed that blood found on Nancy Guthrie's front porch came from her.

Media attention on the case has waned considerably since mid-February, ⁠when the sheriff ​and FBI released surveillance footage of an armed prowler in a ski mask shown tampering with Nancy Guthrie's doorbell camera shortly ​before she was abducted.

But DNA samples obtained from a glove found near her home and resembling the pair the prowler was seen wearing failed to produce a match against known genetic profiles in a national database.

Savannah Guthrie later said her family was still "blowing on ​the embers of hope" that her mother was alive, while also acknowledging that "she may already be gone."