Pima County Sheriff Chris Nanos; Nancy Guthrie.Credit : NBC News/Youtube;Nathan Congleton/NBC via Getty

As the search for Nancy Guthrie enters week four, Pima County Sheriff Chris Nanos says the investigation into her disappearance is “still growing.”

In a new interview with NBC News conducted Friday, Feb. 20, and published Saturday, Feb. 21, Nanos opened up about the status of the high-profile investigation into the 84-year-old mother of Today co-host Savannah Guthrie.

The Pima County Sheriff’s Department (PCSD) launched an urgent search for Nancy on Feb. 1. Asked if the investigation is growing or shrinking three weeks later, Nanos told NBC News, “In terms of leads and working and getting out there, I think that’s still growing, yeah.”

“It’s never fast enough for the sheriff,” he told the outlet. “I want it like you: ‘Come on, guys, let’s go, let’s go, let’s find her.’ But the reality is that I also know that sometimes things take time.”

Nanos also confirmed that there are not currently any names that the department is looking into.

Nancy was last seen on Jan. 31, and the PCSD and FBI believe she was kidnapped in the middle of the night on Feb. 1 by a masked, armed person seen in surveillance footage appearing to tamper with Nancy’s doorbell camera. Investigators are “getting a little closer to identifying some of the other articles” worn by the person, Nanos told NBC News.

“Not just that backpack — like, the shoes, the pants, the shirt or jacket,” added the sheriff.

Nanos also told NBC News that the DNA samples recovered from inside of Nancy’s Tuscon, Ariz., home are mixed — they contain DNA from more than one individual — which has made it more difficult for authorities to run them through databases.

“Our lab tells us that there’s challenges with it,” he told the outlet. “The technology is moving so fast and in such a frenzy that they think some of this stuff will resolve itself in a matter of weeks, months or maybe a year.”

Nanos also shared a message with the Guthrie family, as the search for Nancy enters its fourth week.

“We’re not quitting,” he said. “We’ll find her. Yeah.”

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The sheriff’s message to Nancy’s loved ones comes just days after he ruled out her family members, including all siblings and spouses, as suspects — and explained how.

“We talked to them, we took their phones, we took their computers,” Nanos explained in a statement published on Tuesday, Feb. 17. “I mean, we did everything. We processed their vehicles, we processed their homes. They have been really … They’re victims.”

The FBI previously offered a $100,000 reward for any information leading to Nancy’s recovery or an arrest in the case. On Wednesday, Feb. 18, that reward doubled after the PCSD received an anonymous donation.

Anyone with information about Nancy’s disappearance is asked to please contact 1-800-CALL-FBI (1-800-225-5324) or the PCSD at 520-351-4900.