Nancy Guthrie’s alleged kidnappers are demanding $6 million, it was revealed over the weekend as cops searched a septic tank on her property — and a deadline to deliver the cash by 5 p.m. Monday loomed.

“Multiple ransom notes have been sent out to the media, including one that was sent to us,” said reporter JJ McKinney, whose Arizona outlet KGUN9 was one of three to receive the purported ransom note.

“In the letter, the potential kidnappers demanded that the Guthries pay them $6 million before this Monday,” McKinney said Sunday.

That was the first time the staggering sum demanded for the 84-year-old’s return was revealed since her ransom note emerged days after she went missing from her Tucson home Jan. 31.

The supposed ransomers demanded payment in bitcoin and provided two deadlines in their missive: One warning they wanted $4 million by 5 p.m. local time last Thursday, and another saying that if that line in the sand was missed, they then needed $6 million by the same time Monday — or else.

The note said the elderly grandmother’s life would be in peril if the funds did not arrive by the Monday deadline, KGUN9 reported.

Law enforcement still has not verified whether the note is real.

No proof that Nancy is alive or that the authors have her has been provided, but details from inside her house were included in the note.

Authorities have said that because they have so little else to go on, they have been compelled to take it as seriously as if it were real.

That’s prompted Nancy’s children — celebrity “Today” show TV host Savannah Guthrie and her siblings Annie and Camron — to finally give in and issue a video statement Saturday saying they were ready to pay.

“We received your message, and we understand,” a despondent Savannah said in the video. “We beg you now to return our mother to us so that we can celebrate with her.

“This is the only way we will have peace. This is very valuable to us, and we will pay,” she said.

But some experts remain wary of the ransom note, which specified that the payment be made in the equivalent of US dollars, which some said could indicate the sender was not in America — and could be someone trying to scam the Guthries out of millions as they search for their mother.

“If you’re domestic, why would you ever put ‘USD?’ You put six million,” former FBI agent Michael Harrigan told The Post.

“Why would you use that if you’re a domestic person? That points to somebody who might be potentially outside the country,” said Harrigan, who oversaw the FBI National Academy during his extensive career.

That small detail “lends the potential for this being a scam,” he said.

No other communications from the supposed kidnappers have emerged besides those notes, and investigators pressed on into the eighth day of searching as the deadline approached.

Police were spotted at Nancy’s home Sunday afternoon searching a septic tank behind the house.

Two officers were seen probing the buried tank with a long pole, but eventually left empty-handed.

It remains unclear what they were looking for or whether that tank had been searched before.

The night before, police were seen taking photos inside the Tucson home of Nancy’s daughter and son-in-law, Annie Tommaso Cioni.

The pair were among the last people to see Nancy alive, with Cioni dropping her off at her home on Jan. 31 around 9:45 p.m. after the three of them spent the evening having dinner together.

Cioni was claimed to be a suspect in some unconfirmed news reports last week, though that was later disputed by others.

But police refused to rule Cioni out when asked directly about him at a press conference last week, with Pima County Sheriff Chris Nano insisting everybody in Nancy’s orbit was being looked at.

“We’re actively looking at everybody we come across in this case, we would be irresponsible if we didn’t talk to everybody,” Nanos said.

“Everybody’s still a suspect in our eyes,” he added. “The family’s been very cooperative; they’ve done everything we’ve asked of them.”