The Pima Country Sheriff's Department and the FBI Guthrie Task Force were recently made aware of the possible ransom note as well as social media comments related to it, a statement from the sheriff's department said on Monday.
Officials did not immediately release details on how they found the note or what was in it but said investigators were working to determine its credibility.
In July, authorities released two ransom notes initially sent to a Tucson television station days after the mother of Today show host Savannah Guthrie went missing on February 1 from her home outside Tucson.
The first note, sent February 2, said Nancy Guthrie was safe but scared and would be released unharmed once her captors received $4 million in cryptocurrency.
The second note, sent February 6, said the 84-year-old died shortly after she was kidnapped.
No suspects have been arrested in her disappearance. The investigation is dogged by unanswered questions, such as how many people are believed to have taken part in her abduction and whether she was taken by someone who knew her.
The FBI had also released surveillance videos of a masked person who authorities said appeared to tamper with a camera on Nancy Guthrie's front porch and block it with a plant.
Nancy Guthrie's family has shared their pleas for information in videos posted to social media.
In July, Savannah Guthrie said her family was living an unending nightmare nearly six months after her mother's disappearance and pleaded with her kidnappers to: "Tell us where to look for her".
She said her family would never stop looking.
"The world can be a cruel and unforgiving place, and I know that you have tried to do things the right way," she said.
"So I'm asking you, begging you, to do the right thing now. Make the right choice. Help us find her. Tell us where to look for her. I truly believe it is never too late to do the right thing."
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