The reward was increased on Thursday from $US50,000 ($A70,000) to $US100,000 ($A140,000).
The agency also released new visuals, including photos of a backpack the suspect could be seen wearing in video footage, and an updated description of the suspect following "forensic analysis" of footage from a doorbell camera.
The suspect was described as a male, between 175 and 177cm and was seen wearing a black, 25-litre backpack, the FBI said in a statement. The agency identified the backpack as an Ozark Trail brand Hiker Pack.
Nancy Guthrie, 84, was first reported missing on February 1 in Arizona.
Guthrie's disappearance has captured the nation's attention with the FBI saying it had received more than 13,000 tips from the public.
The Guthrie family, including NBC Today co-anchor Savannah Guthrie and her siblings, has released multiple videos pleading for their mother's return.
"We believe she is still out there," Savannah Guthrie said on Tuesday in a post on Instagram with video footage of the suspect at her mother's home. "Bring her home."
Meanwhile, an Arizona sheriff is blocking FBI access to key evidence in the investigation, impairing its ability to assist in the probe, a US law enforcement official with knowledge of the case told Reuters on Thursday.
The FBI asked Pima County Sheriff Chris Nanos for physical evidence in the case, including a glove and DNA from Nancy Guthrie's home, to be processed at the FBI's national crime laboratory in Quantico, Virginia, but Nanos has insisted instead on using a private lab in Florida, the official said.
Outsourcing forensic analysis to a Florida contractor, effectively denying the access of the Federal Bureau of Investigation to crucial evidence in the case, is delaying the FBI ability to assist in the case, according to the official.
A spokesperson for the Pima County Sheriff did not immediately respond to Reuters' request by email seeking comment.
The Pima County sheriff has primary jurisdiction over the case, and FBI assistance must be officially requested by the county, otherwise the FBI is legally precluded from taking part in the investigation. The official said the county has spent some $US200,000 ($A280,000) so far to send evidence in the Guthrie case to the Florida lab the county contracts with.
"It risks further slowing a case that grows more urgent by the minute," the official told Reuters, citing unspecified "earlier setbacks" in the investigation.
The official also criticised the sheriff for not seeking FBI assistance in the investigation sooner.
"It's clear the fastest path to answers is leveraging federal resources and technology. Anything less only prolongs the Guthrie family's grief and the community's wait for justice," the official said.
Signs of friction between the FBI and sheriff's department emerged as the search for Nancy Guthrie stretched into its 12th day, as investigators intensified their search for clues in the presumed kidnapping for ransom.
22°C