Investigators in the Nancy Guthrie case should pay close attention to the “fingerprint” hidden in her kidnapper’s alleged ransom notes.

“Ransom communications have a ‘fingerprint’ to them,” retired FBI agent Jason Pack told Page Six Tuesday.

Key elements to pay attention to are the word choice, tone and how someone structures a demand, Pack explained.

“If the first two [ransom notes] read like the same person wrote them and everything that followed reads differently, that tells the task force something meaningful about who they’re actually dealing with versus who decided to insert themselves into the story once it went international,” he said.

Pack said that investigators will be able to deduce “meaningful” information if the notes are “authentic.”

“The first note apparently contained specific operational details that weren’t public at the time,” Pack explained. “What she was wearing. The damaged floodlight. Those aren’t details you get from a news report.”

“Someone was likely there,” he continued. “Based on what’s been reported, the language and tone of those first two notes compared to everything that came after is where the real analytical work is happening right now.”

Pack weighed in after reports surfaced that officials searching for Savannah Guthrie’s 84-year-old mother made a major mistake early on in the investigation.

Sources told Air Mail that the Pima County Sheriff’s Department and the FBI allegedly bungled a chance to find Nancy in early February. The source claimed that a potentially credible ransom note was sent hours after Nancy was reported missing from her Arizona home on Feb. 1.

The note, which described accurate details about Nancy’s outfit at the time of her disappearance, and the damaged floodlight in her Tucson-area backyard, asked that $4 million in Bitcoin be paid to a Bitcoin address by Feb. 5.

However, investigators chose not to pay the full sum and instead sent just sent $152 to the Bitcoin address in the hopes that they could trace the money if it was ever to be cashed out. (As of Tuesday, that balance has not been cashed out.)

Days later, an alleged ransom note from the same I.P. address as the first was allegedly sent on Feb. 6, but the tone was “sputtering,” “labored” and “less confident” than the first note.

The purported kidnapper allegedly apologized for Nancy’s death and requested that the ransom be paid in exchange for the return of the senior citizen’s body.

“The task force had to throw up their hands in dismay and concede that … nothing in the collections of carefully analyzed notes had gotten them any closer to finding the culprits. … It was too late,” the outlet claimed, adding, “The damage had been done.”

Pack told Page Six on Tuesday that the public may not have the full picture regarding why investigators chose not to pay the ransom when they had the opportunity.

“The task force had information the public doesn’t have and likely never will,” Pack said. “The family was part of those conversations. Everyone involved made the best decisions they could with what they knew at that moment, not with what we think we know five months later reading news reports.”

The retired fed cautioned the public not to question the investigators’ decisions based on public information.

“From what’s been reported publicly, it sounds like a mousetrap that didn’t spring,” Pack said. “For whatever reason, the other side didn’t respond the way investigators hoped.

“Based on what we know publicly, that’s not a mistake. That’s sometimes how these things go and the full picture may look very different than what we’re seeing from the outside.”

Nancy was last seen alive when her family dropped her off at her home after dinner on Jan. 31, and she was reported missing the next day. Cops concluded that Nancy was probably taken from her home in the middle of the night against her will after discovering a trail of blood outside her home.

In February, police released a horrifying video of a masked and armed person breaking into her home on the night she went missing. Investigators also reportedly conducted DNA testing on a hair sample. However, no official suspects have been named.

Multiple ransom notes demanded Bitcoin payments in exchange for Nany’s release, prompting Savannah and her family members to announce that they are willing to make a payment but needed proof of life.

Savannah — who made an emotional return to the NBC show on April 6 — tearfully addressed the latest update about the contents of the ransom notes on the “Today” show Tuesday.

“I just wanted to take the opportunity to really ask people and really beg people to come forward because somebody knows something,” Savannah, 54, said.

“This is a news story today that is on your radar, but this is the live my sister, [Annie Guthrie], lives, that I live, that my brother, [Camron Guthrie], lives, that our extended families live, that our children live every day,” she added.