As a real kidnapping investigation unfolded around a missing 84-year-old woman, prosecutors say, a Southern California man saw an opening to exploit her family's fear. On Thursday, he admitted it.

The plea

Derrick Callella, 42, of Hawthorne pleaded guilty in federal court in Arizona to two felony counts — sending a ransom demand across state lines and using a telecommunications device to threaten or harass, NBC News reported. Under the plea agreement, he faces five years of probation and is scheduled to be sentenced Sept. 10. The charges carry a maximum of two years in prison and a $250,000 fine.

Callella admitted that on Feb. 4 he contacted members of Nancy Guthrie's family by text and phone, referencing a bitcoin transfer, according to the Los Angeles Times. Prosecutors said he had found the family's contact information online and was following news coverage of the case, knowing an earlier ransom demand had already been made.

Not connected to the disappearance

Authorities have been clear on one crucial point: Callella is not accused of any role in Guthrie's disappearance itself. He was, in prosecutors' description, an imposter trying to take advantage of an ongoing tragedy — one of several ransom communications the F.B.I. has had to sort through, some dismissed as hoaxes and at least one still being evaluated as potentially genuine.

The underlying case

Nancy Guthrie, 84, the mother of NBC Today co-anchor Savannah Guthrie, was reported missing from her home in the Catalina Foothills near Tucson, Ariz., on Feb. 1. Investigators found evidence at the home indicating she had been taken against her will, and the F.B.I. has been investigating her disappearance as a kidnapping-for-ransom case. Months later, her whereabouts remain unknown and no one has been charged in the abduction.

The guilty plea closes one strange chapter of a case that has drawn national attention — the prosecution of a man who added to a grieving family's anguish from a distance — even as the central question, what happened to Nancy Guthrie, goes unanswered.