Nancy Guthrie Update: Former FBI Agent Shares New Theory on Disappearance

It has been exactly three weeks since Nancy Guthrie was reported missing. Law enforcement continues to pursue thousands of leads in the search for the 84-year-old Tucson, Arizona resident, who is presumed kidnapped.

Authorities have struggled to come up with a motive for someone abducting Guthrie. Now, a former FBI agent has shared his own possible theory on what motivated the suspected perpetrator.

'Personal grievance' a possibility

Jonny Grusing, a 25-year veteran of the FBI's Denver Division, spoke with Fox News Digital this weekend regarding Guthrie's case.

Going off public information only, the longtime investigator and published author hypothesized that an unknown "personal grievance" may be the explanation for the kidnapping of Today co-host Savannah Guthrie's mother.

"It's hard to be an expert in human behavior because it's so unique to that person," said Grusing.

"You know, I'm just trying to use the experiences of different cases and trying to apply any sort of logic to this in the hopes that someone from the public who has thought it might be someone they know whether it's his family or whether now it's a coworker or friend or associate or whatever, to put that one puzzle piece together that says, 'Yes, and now I think it could be him.'"

What evidence led Grusing to this theory?

Grusing cited a number of factors, including the actions of the subject in the surveillance video released by the FBI earlier this month.

"The first thing he does is with his glove, and with his glove, it doesn't look like he's trying to take [the camera] off," Grusing said. "It looks like he's trying to cover it with his right hand.And then he looks down, he looks around, and he gets the branches, and he puts the branches up in front of it."

"Is there a chance, since we don't have audio, that he is either knocking on the door loudly or that he has pressed the ring doorbell, [that] he's trying to get Nancy to answer the door, and he's shielding himself from being seen as a masked person, so she will, in her confusion, open the door?" Grusing continued.

If this is the case, Grusing said the situation would not fit the home robbery gone wrong theory, since the suspect seemingly wanted to lure Nancy Guthrie onto the porch for a confrontation.

Grusing also said kidnapping for ransom seems unlikely since none of the ransom notes were sent directly to the Guthrie family.

Guthrie latest: DNA testing hits possible snag

On Saturday, Pima County Sheriff Chris Nanos issued an update on DNA found in Nancy's home, calling it "mixed," meaning there is material from more than one individual.

This makes DNA testing more difficult when entering the findings through national databases.

“Our lab tells us that there’s challenges with it,” Nanos said before reiterating that "we're not quitting" and his department remains steadfast in locating Nancy, who was last seen on Jan. 31.