A man who claims to know who kidnapped Nancy Guthrie says he has video of her killers from the day she died that would 'deliver them on a silver platter'.

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. 

Media outlets began receiving cryptic ransom notes in the weeks that followed, some claiming to know where she was and demanding millions of dollars.

Police and the FBI combed through them for leads, but there is still no trace of the missing grandmother almost five months later.

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

They added that if the ransom was not paid by February 9, this would increase to $6 million, 'or else'.

Then an '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 $4 million. 

Reports of those emails did not emerge until this week, and another arrived at gossip site TMZ in response - with new claims.

The writer claimed there were two kidnappers, and he had video of 'the main guy' with Guthrie on the day she died stashed on a hidden phone. 

'I have a phone stashed in a secure location guaranteeing both the information it stores and the safety of the phone,' TMZ said the email read.

'What it contains is my definition of delivering them on a silver platter, a short video of the main guy with nancy the day that was probably her last, pictures of both involved, names and addresses and age.' 

The email explained the 'location [is] easy to access if you know where it is' and he would reveal it along with the password in exchange for a huge Bitcoin payment.

He also insisted he was not the same person who told volunteer group Buscando Corazones Nogales that Guthrie was buried near the US-Mexico border.

The tip prompted the organization to search the area near Nogales that it specified, but found nothing.

'I am not the idiot who recently called in a tip about her burial site in Mexico,' the email to TMZ read. 

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

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

The fact that they apologized 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.

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.

Pima County Sheriff Chris Nanos had previously said no footage was available because Nancy did not have a subscription to Google's video storage service, which typically keeps recorded clips in the cloud. 

The FBI managed to obtain the doorbell camera footage days by accessing residual data stored on backend servers.

Additional video released by officials showed the individual attempting to cover the lens with a clenched fist, then searching the area around the door, ripping a bunch of flowers from the entranceway and waving them at the camera.

Another clip shared by the FBI showed the individual hunched over as they made their first approach to the door, bowing their head.

No suspects have been publicly identified in the case.

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'.

Authorities believe Nancy was kidnapped, abducted or otherwise taken against her will after finding blood near her front doorstep.

The Guthrie family is offering a reward of up to $1 million for information leading to Nancy's recovery.