"The Pima County Sheriff's Department continues to work closely with the FBI as investigators follow up on leads, review information, and pursue the facts surrounding this case," a spokesperson for the sheriff's department said.
The BBC has contacted the FBI, which has with Pima County spent months investigating the elder Guthrie's mysterious abduction from her home near Tucson, Arizona.
The BBC's US partner CBS and other outlets reported on Monday that law enforcement had requested that media hold off publishing details of the notes while they investigated Guthrie's disappearance.
Nancy Guthrie vanished after being dropped off at her home by relatives on 31 January. Concern grew when she did not go to a friend's house to watch a virtual Sunday church service the next morning.
An initial ransom note was sent the day after she went missing and demanded millions in bitcoin for her release.
It gave specific details about her home, as well as details about her bedroom and the home's surroundings, investigators told CBS.
The note was reportedly addressed to Savannah Guthrie, who is a co-anchor for the NBC's Today show. It was sent to multiple media outlets, including a local TV station which reportedly adhered to a police request not to disclose its contents.
It has only now emerged that a second note - sent on 6 February - used language similar to that of the first one, but did not include any demands, instead apologising for her death and stating it was inadvertent.
After the two notes, Savannah Guthrie and her siblings released a video addressed to the kidnappers.
"We received your message, and we understand," the NBC host said. "We beg you now to return our mother to us." She said the family "would pay".
Authorities and Guthrie's family repeatedly issued public warnings that the elderly victim was in poor health and was without critical medication.
As the search continued, authorities released images of a masked person who was seen in security camera footage outside her home.
The Guthrie family had been offering a $1m (£760,000) reward, in addition to $100,000 pledged by the FBI, for information leading to Nancy Guthrie's return.
On 24 February, Savannah Guthrie said they would keep hoping, even though they "know that she may be lost, she may already be gone".
In a March interview with NBC, Guthrie said several ransom notes were sent, and she believed some of them were bogus. She also told the network her family believed the two initial notes were authentic.
Guthrie stepped away from NBC's Today show for more than two months while the investigation was underway. She returned to the programme in early April as the search continued.
