The Nancy Guthrie reward money has been doubled as the search for Savannah Guthrie's missing mom continues

The 88-CRIME tip line has received a $100,000 pledge from an anonymous donor to bring the total reward money to $102,500 in addition to the $100,000 which is being offered by the FBI.

It comes as Pima County Sheriff Chris Nanos revealed they are still combing over footage taken by a doorbell camera which shows an unidentified masked man on Nancy's doorstep.

He explained they are hunting for clues relating to the rucksack the man was wearing as well as trying to track down more details about his gun and its holster.

Speaking to NBC, nanos explained: "That backpack, we could positively now identify as a backpack that is sold at one place only. That's Walmart.

"So we're working with our Walmart managers all across the state to try to find out how many sales were there of that backpack in the last 20, 30 days, the last 60 days.

"And can we do something with that? Can we break it - maybe we'll find a credit card or a bank card. Maybe we'll find a video of the guy walking in."

He added of the man's weapon: "We know he had a gun. We know he had a holster that had some pretty unique characteristics.

"We can't quite identify it yet, but that's being worked on. So naturally, we go to our gun shops everywhere and say: 'Have you seen this guy? Can you help us identify this weapon? Can you help us identify this holster?'"

In a subsequent interview with NBC, Nanos insisted authorities are not going to give up the search, he added: "As long as we have the ability to chase a lead, it's not cold. We're not going to give up. We're going to find Nancy, and we're going to find out who did this."