Derrick Callella, 42, pleaded guilty to two counts of harassment by telecommunications device, marking the only criminal conviction to date stemming from the disappearance of 84-year-old Nancy Guthrie from her home in Tucson, Arizona, five months ago.
The charges carry a maximum penalty of two years in prison and a $US250,000 fine. But the plea agreement with prosecutors calls for Callella, a resident of Hawthorne, California, to serve five years on probation, according to a US Attorney's Office spokesperson.
Formal sentencing was set for September 10.
The plea deal, entered in US District Court in Tucson, leaves Guthrie's fate unknown and the underlying kidnapping case as yet unsolved.
Guthrie, in frail health with limited mobility, was last seen alive on January 31 at her home. A friend contacted family members the next day, when she did not show up to church as expected, and relatives entering her home found her gone.
A search of Guthrie's residence showed she had left behind essential items such as her wallet, mobile phone, hearing aid and medication. Blood found on Guthrie's front porch was hers, DNA tests confirmed later.
He was arrested on February 5, four days after Guthrie was reported missing. By then, according to court records and FBI public statements, local media had received a ransom note on February 2 demanding payment in bitcoin and setting deadlines for payment.
In entering his guilty plea, Callella admitted that he called and sent text messages to Guthrie's family on February 4 inquiring about a bitcoin transfer, and "he acknowledged that he knew an earlier ransom demand had been made," the US Attorney statement said.
"Callella also admitted that his actions were meant to harass the family by seeking information about the investigation into the missing person's disappearance," the statement said.
The guilty plea came a day after the FBI field office in Phoenix said on X that investigators have received "several" ransom notes in the course of the investigation, some deemed to be "extortion attempts without legitimacy," others being treated as potentially genuine.
The FBI said Guthrie's disappearance continues to be investigated as a kidnapping-for-ransom case.
Last week, celebrity news site TMZ.com reported receiving an email from someone claiming to know the identities of the abductors and to have video of the "main guy" involved in her kidnapping, as well as of Guthrie on the day she died.
An FBI official, speaking on condition of anonymity, told Reuters the latest TMZ letter and two ransom notes reported by media in February have since been deemed by investigators to not be credible.
