In a new Instagram post on Thursday, Savannah Guthrie shared an old home video from her childhood, showing her and her sister with their mother, Nancy Guthrie. The video is followed by a still photo of all three siblings with Nancy Guthrie.

"[O]ur lovely mom," Savannah Guthrie wrote in the caption. "we will never give up on her. thank you for your prayers and hope."

The Pima County Sheriff's Department is now asking Nancy Guthrie’s neighbors to check their home security cameras for footage from an earlier timeframe than the early morning of Feb. 1, when she was likely abducted.

Investigators on Thursday asked homeowners for video footage from Jan. 11 between 9 p.m. and midnight. They did not specify why.

They also asked for footage from Jan. 31 between 9:30 a.m. and 11 a.m. Authorities said there was a suspicious vehicle on a nearby street around 10 a.m.

The request came through the app Neighbors.

-ABC News' Aaron Katersky

The Pima County Sheriff’s Department and the FBI said on Wednesday they are continuing to follow up on all 18,000 calls to investigators related to the disappearance of Nancy Guthrie.

Up to 4,000 calls to investigators have been made since the FBI released doorbell-camera images and video of an "armed individual" apparently tampering with a security camera at the front door of the 84-year-old woman's house near Tucson, Arizona, authorities said.

"We thank the media and the public for the thousands of tips submitted through the various tip lines," the sheriff's office and the FBI said in a joint statement.

Several hundred detectives and agents are assigned to this case, and authorities said, "You will continue to see increased law enforcement activity throughout Pima County as the investigation expands."

-ABC News' Aaron Katersky

The individual detained for questioning in connection with the disappearance and abduction of Nancy Guthrie had been on the radar of the FBI and Pima County Sheriff’s Department before Tuesday's public release of the images of a masked subject at her front door, law enforcement sources told ABC News.

The individual, who identified himself as Carlos, was detained for questioning in Rio Rico, Arizona, south of Tucson, after the FBI identified him by using a series of electronic investigative tools, including cell phone usage information and traffic data, the sources said, without specifying the exact techniques.

A local judge from Santa Cruz County, Arizona, signed the warrant for the search of Carlos' house, which was carried out overnight.

Carlos was questioned for several hours before being released without charge. He is under no law enforcement restrictions.

Separately, investigators are studying every pixel of the new video the FBI released publicly on Tuesday to try to find any identifying feature, including the apparent weapon and characteristics of what the subject was wearing, according to the sources.

-ABC News' Aaron Katersky and Trevor Ault