Police found "no evidence" the suspect visited Nancy Guthrie's home before the day of her disappearance.

The 84-year-old mother of Today show star Savannah Guthrie went missing near her home in Tucson, Arizona on February 1, and Pima County Sheriff Chris Nanos has dismissed reports that a masked man seen on doorbell camera footage was spotted at the house at an earlier date.

Nanos told PEOPLE magazine: "There is no evidence to support that… and there is no evidence to support it was all on the same day.

"It is speculative at best and remains part of an ongoing investigation."

However, Nanos conceded that the masked suspect may have returned on a different day, as he's seen without a backpack or gun in one of the clips and photos shared by the FBI on February 10.

He added: "It could be accurate. We just don't have evidence to support when each image was taken…

"There is no date or timestamp and we are still investigating it."

In a separate statement, police reiterated that "conclusions will be guided by verifiable evidence" rather than speculation.

The Pima County Sheriff's Department commented: "We are aware that doorbell images released earlier in the investigation depict a suspect in different stages of attire, including with and without a backpack.

"There is no date or time stamp associated with these images. Therefore, any suggestion that the photographs were taken on different days is purely speculative.

"This remains an active and ongoing criminal investigation.

"As with any investigation, conclusions will be guided by verifiable evidence and established facts. Speculation, without factual support, does not advance the investigative process."

Last week, Nanos revealed the sheriff's department and FBI are still combing over footage taken by a doorbell camera which shows an unidentified masked man on Nancy's doorstep.

Speaking to NBC, he 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."