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 on Tuesday.
"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".
Nancy 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.
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 $US1 million ($A1.4 million) reward being offered by her family, whom she described as being in "agony" since her mother vanished.
