A drone view of Nancy Guthrie's house in Tucson, Arizona, on Sunday.

Investigators are likely ramping up their efforts in the search for missing 84-year-old Nancy Guthrie, as the deadline in a purported ransom note draws closer, former SWAT team captain Josh Schirard said Sunday.

“If this is real, they’re on the clock, and that clock is ticking short, so anything that they can do has to be done and has to be done now,” Schirard said, noting investigators have returned to the homes of Nancy Guthrie as well as her daughter, Annie Guthrie.

Savannah Guthrie and her siblings released a video Saturday night saying they “will pay” for their mother’s release. Purported ransom notes have demanded millions in Bitcoin in exchange for her return.

“I think we still have no valid two-way communication with this possible hostage taker and the author of these ransom notes,” Schirard told CNN’s Fredricka Whitfield. “The family putting out these messages has been the only way that they’ve tried to communicate back in order to establish some form of authenticity, so that they can get a proof of life and know that Nancy is still okay.”

With one deadline from the ransom notes passed and another approaching on Monday, “both sides are probably getting a little desperate” to start communicating, he said.

“When we look historically, we generally don’t see deadlines that are this far out when we’re talking about dealing with hostage takers,” the former SWAT captain said.

Authorities have yet to authenticate the ransom notes.

“Now the video we saw last night from the family was very cryptic, but very directed to that ransom note author,” he said. “So there may be some things in that note that obviously law enforcement isn’t releasing that may be lending to its credibility and authenticity, and this was the response for that.”