One of the disturbing ransom notes received by Nancy Guthrie's family included a chilling apology for accidentally killing her, according to a new report. 

A bizarre blackmail letter sent February 6 offered to return the 84-year-old's body in return for money, a source familiar with the investigation told Air Mail. 

The memo followed an earlier note from the same sender, which correctly described what Guthrie was wearing on the night she was abducted, and included information about a damaged floodlight in her backyard. 

Guthrie, who is the mother of NBC News Today anchor Savannah Guthrie, was kidnapped from her home in Tucson, Arizona, on the night of February 1. 

Initial messages from the sender said Guthrie was 'safe but scared', and agreed to return her if the family paid $4million in Bitcoin by 5pm on February 5. 

They added that if the ransom was not paid by February 9, this would increase to $6million, 'or else', according to Air Mail, a digital weekly newsletter. 

The 'apology' note, which came from the same IP address as these initial, seemingly credible emails, arrived at the Guthrie family's home on February 6. 

It said that Guthrie had been accidentally killed, and offered to deliver her body back to the family for a sum of $4million.  

The following day, Savannah posted a heartbreaking clip on Instagram, sitting next to her brother, Camron, and sister, Annie.

'We beg you now to return our mother to us, so that we can celebrate with her,' the TV host pleaded in the video posted to her Instagram page.

'This is the only way we will have peace. This is very valuable to us, and we will pay.'

Savannah has since said she believes those ransom notes were real.

As the investigation into Nancy's disappearance continues, federal authorities are re-examining the letters for any clues about the captor's identity, according to Air Mail.

They note that the ransom letters show that the captor is articulate and knowledgeable about the intricacies of cryptocurrency.

The fact that they apologised also suggests the kidnapper is not a cold-hearted cartel member as they had originally feared, but was rather a local opportunist.

Authorities are also probing the possibility the abductor may have had an accomplice – a masked man who was seen trying to remove Nancy's Nest doorbell camera on the night of her disappearance, according to Air Mail.

Federal investigators say the masked man's bumbling attempt to remove or cover up the doorbell camera does not seem to align with their profile of the articulate and cunning suspect, the outlet reports.

No suspects have been publicly identified in the case.

According to reports from Page Six entertainment news outlet, investigators have uncovered only limited physical evidence, including a single strand of hair and a glove found near Nancy's home.

The DNA evidence recovered near the scene has since undergone extensive testing at an FBI crime lab after initially being processed by a private laboratory in Florida.

But Arizona's Pima County Sheriff Chris Nanos said 'the testing moves at a snail's pace'.

He noted in an interview with People magazine that local authorities have already combed through 'thousands and thousands' of hours of video footage gathered from traffic intersections and Ring doorbell cameras across the Tucson area.

In an earlier interview with KOLD, a news station in Tucson, the sheriff revealed that investigators possess information they are deliberately withholding.

'It's not done because we got to keep it secret,' Nanos explained. 'It's done because we got to protect our case.'

The sheriff then maintained that he remains convinced detectives will eventually identify the masked suspect seen on surveillance footage outside Nancy's home.

'I believe at some point in time, we will make an arrest in this case,' he said. 'And whoever that individual is, that individual will have a right to a fair and impartial trial.'