As the search for Nancy Guthrie stretches on to day 19, a retired sergeant speculates the case is taking a major toll on police handling the case. "Well, they're exhausted," Troy Hillman, retired cold case homicide sergeant with Phoenix Police Department, told Us Weekly exclusively. Although Hillman is not part of the ongoing investigation, he gave Us insight as to how officers have likely been coping behind the scenes as they work around the clock in hopes of reuniting 84-year-old Guthrie with her loved ones. "My guess is they've been running on [no sleep], they've been away from their families and [are] running on fumes," Hillman explained. "They're frustrated." The Pima County Sheriff's Office and Federal Bureau of Investigation are currently sifting through clues following the public release of new images and doorbell camera footage, which shows a masked figure approaching the door of Guthrie's home in Tucson, Arizona, on the night of her disappearance. Retired Phoenix Sergeant Breaks Down New Photo Clues in Nancy Guthrie Case The FBI has described the suspect as a man that is around 5-foot-9 to 5-foot-10 and of average build. He appears to have facial hair and seems to be wearing an Athletic Works Fusion Knit jacket and an Ozark Trail Hiker backpack as well as a semiautomatic handgun in a holster around his waist. "They want to get the bad guy in jail, and then be able to kind of resume their lives or whatever normality they have," Hillman, author of Chasing Down the Zombie Hunter, told Us. "It's just a high level of frustration, but I can tell, having worked in some of those investigations, we're human beings, and we're ultimately tied to the family. We want to get her back and not screw up the investigation." Today show anchor Savannah Guthrie continued to plead for her mother's safe return in a video shared on Sunday, February 15, noting, "It's never too late to do the next right thing." FBI More recently, Us learned the reward doubled for any information leading to Nancy's location or the arrest and conviction of anyone involved in her disappearance. She was last seen being dropped off at home on January 31 and was reported missing the following day on February 1. An update from the Pima County Sheriff's Office confirmed that 88-CRIME received an anonymous $100,000 donation "allowing the program to increase the reward for this case to $102,500." That dollar amount is in addition to the $100,000 being offered from the FBI, bringing the total reward to $202,500. Sheriff Reacts to Theories Nancy Guthrie's Family Could Be Involved in Case As the investigation continues, Hillman said the goal is ultimately "not to have a misstep, because we're hurrying, and obviously, protect the integrity of the investigation, so that there's not a technicality that a defense attorney can use down on the line." Earlier this week, a glove found one and a half miles away from Nancy's residence produced no DNA matches after being added to the national database. Hillman predicted what lies ahead, telling Us, "You just let it sit and code and hope that you would get a hit eventually." Going forward, Hillman said it's likely "they'll swab everybody that's been at the house and in contact, then they'll exclude." He pointed out, "There could be somebody random," and authorities will want to pursue all leads before crossing them off the list.