Chilling Claim: A ransom letter says it knows who ...

The Arizona sheriff whose department, along with the FBI, is investigating the kidnapping of Nancy Guthrie, is speaking out after a new demand letter claimed to have evidence tying two people to the case.

TMZ reported on Friday, June 26, that the person who sent the email to the outlet said they have a phone stashed in a “secure location” containing what they claimed was a video of the “main” suspect with Nancy on the last day they believe she was alive.

The sender added that the phone also contains pictures, names, ages and addresses of “both involved,” as well as a bitcoin address. TMZ reported that it forwarded the note to the FBI. The federal agency declined to comment when reached by PEOPLE on Friday.

Responding to the letter, Pima County Sheriff Chris Nanos expressed doubt about its legitimacy during an interview on the Buckmaster Show, which aired Friday on Arizona radio station KVOI AM 1030.

“I think the FBI has done a number of arrests for false or fake ransom notes,” Nanos said. “It’s a shame that that happens, but I think we’re looking at another one of those today.”

He added that it’s “good” people have “great interest” in the case because it helps investigators, but then it “really gets abused” with those who “call in fake ransom notes.”

The PSCO told PEOPLE on Friday that the Guthrie investigation remains “active and ongoing.”

Authorities believe 84-year-old Nancy, the mother of Today show co-host Savannah Guthrie, was taken from her Tucson, Ariz., home against her will in the early morning hours of Feb. 1. Shortly after her disappearance, two separate ransom notes were sent days apart to local media outlets, which the FBI and PCSO confirmed at the time.

The first note reportedly demanded millions of dollars in Bitcoin in exchange for Nancy’s safe return, while some of the contents of the second one remained undisclosed until this month, sparking questions about whether Nancy is still alive.

In the second note, the alleged kidnapper claimed that Nancy died unintentionally shortly after being kidnapped and reportedly expressed regret over her death, multiple sources close to the investigation told CNN and NBC News.

It also did not include an apology or any demand for money in exchange for the return of her body, the outlets reported.

While Nancy’s whereabouts remain unknown and “mixed DNA” recovered from her home is still undergoing forensic testing, Nanos has said he remains hopeful the case will be solved.

“My team, I’ve said all along, they’re gonna solve this,” Nanos told PEOPLE in May. “I fully, 100% believe that.”