As the investigation into the disappearance of Nancy Guthrie nears the end of three weeks, the FBI is continuing to press forward with an analysis of cell phone data, video, and interviews, an agency official said Friday.

That work includes the challenging effort of retrieving any additional images of the suspect from the back-end of Guthrie’s video security system, the official said.

At the agency’s Tucson field office, the command post remains fully active, the official said. Some personnel are being rotated through the post to keep “fresh eyes” on the case reviewing evidence, a second official said.

Pima County Sheriff Chris Nanos said in an interview Friday that he believes investigators are getting closer to identifying some of the articles of clothing worn by the masked man captured on Guthrie’s security camera and released last week by the FBI.

His backpack — a 25-liter Ozark Trail Hiker Pack — was previously identified as a product sold exclusively at Walmart.

In addition to trying to identify the suspect’s pants, shoes and jacket, Nanos said, investigators are visiting gun stores and asking about the holster he was seen wearing and the firearm he appeared to be carrying.

The department is also awaiting an analysis from partial DNA found at Guthrie’s home.

Asked about his message to the Guthrie family, he said: “We’re not quitting. We’ll find her.”

Guthrie, the mother of “TODAY” co-anchor Savannah Guthrie, was last seen on the evening of Jan. 31, when she was dropped off at home shortly before 10 p.m., according to a timeline provided by Nanos.

Her doorbell camera disconnected at 1:47 a.m. on Feb. 1. Her pacemaker disconnected from her phone less than an hour later. Her family reported missing later that day.

Nanos has said investigators believe she was taken in the middle of the night. Authorities have not identified a possible motive, nor have they identified any possible suspects.