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That is "purely speculative," the Sheriff's Department said.

"We are aware" that doorbell images released during the investigation "depict a suspect in different stages of attire, including with and without a backpack," the department said in a written statement Monday night. "There is no date or timestamp associated with these images. Therefore, any suggestion that the photographs were taken on different days is purely speculative. ...

"Speculation, without factual support, does not advance the investigative process," the Sheriff's Department added. 

ABC News was first to report earlier Monday that: "The masked man who is suspected of the abduction of Nancy Guthrie than Feb. 1, the night police believe she was kidnapped, sources familiar with the investigation told ABC News.

"The image the FBI released of the suspect at her front door, without a backpack, was captured by her Nest doorbell camera on a day before the suspected abduction, the sources said. ... The FBI said the images of the suspect with the backpack on are from 'the morning of her disappearance,' Sunday, Feb. 1."

ABC News said the FBI and the Pima County Sheriff’s Department had declined to comment on the sources' claims. 

CNN followed ABC's report, saying: the masked person seen on the doorbell camera footage of Nancy Guthrie’s Arizona home the morning she is believed to have been kidnapped had also appeared at her doorstep on another night."

The statement Monday night from the Sheriff's Department did not clarify how investigators were previously able to say the images were from the night Guthrie went missing, if they did not have date or time stamps. 

In releasing the surveillance images and video on Feb. 10, the Sheriff's Department had said the images were recovered from a Nest camera recording at Guthrie's home. Investigators also said then that the video "was recovered from residual data located in backend systems."

The FBI and Sheriff's Department said then that they had been working with private-sector partners to recover video footage or images that may have been “lost, corrupted, or inaccessible� after video cameras at Guthrie’s home were damaged or removed.

The images that were recovered, and released on Feb. 10, showed a man wearing a ski mask and a holstered gun, who used vegetation from Guthrie's yard to try to cover up the doorbell camera and appeared to be tampering with it. 

Investigators have said they have not identified a suspect in the abduction of Guthrie, 84, a retired University of Arizona communications professional who is the mother of NBC "Today" show host Savannah Guthrie. 

They are not ruling out the possibility that more than one person was involved, they have said.