In the devastating aftermath of the Sept. 11, 2001, terrorist attacks, more than 300 search-and-rescue dogs and their handlers worked tirelessly for months at ground zero. These "four-legged heroes," as the 9/11 Memorial & Museum refers to them, played a critical role in the recovery efforts at the site of the deadliest terror attack in U.S. history.

Robert Schnelle, a member of the New York Police Department's K-9 Unit, was among the first responders. He and his dog, Atlas, were present when the first plane struck the North Tower of the World Trade Center and witnessed the second plane hit the South Tower. Shortly after, the South Tower collapsed while they were at the scene.

Schnelle and Atlas remained at ground zero for eight months, working 12-hour shifts. "We did that straight through till May 30, [2002], until they shut it down and said that the recovery is now over, the site's now closed, ground zero," Schnelle recalled. "That was a very traumatic day for a lot of us, because we had been there so long, and, you know, you never want to just give up."

Dr. Cynthia Otto, a veterinarian who traveled from Pennsylvania to assist with the rescue efforts, noted that it quickly became apparent the dogs' mission would largely shift to recovery. "All of these dogs were trained to find live humans, but it became clear that they started to recognize human remains, and they didn't do their normal response," Otto told ABC News. "That was just the most devastating part of the whole process."

Despite this heartbreaking reality, the dogs proved invaluable in locating human remains. Their keen sense of smell and ability to navigate treacherous debris allowed them to identify victims in spaces inaccessible to humans, aiding in the identification of nearly 3,000 people killed in the attacks.

Beyond their search capabilities, the dogs also served an unintended therapeutic role for those working at ground zero. "One of the things that has really stuck with me is that the dogs provided so much more than just the scent detection," Otto said. "They provided some sort of source of comfort for all of us that were working there, and it really made a difference."

Otto shared a memory of a dog named Logan interacting with firefighters taking a break amidst the rubble. "They were sitting on a beam on the pile, and they reached out and petted [Logan], and you could just see the light go from blank to [having] light in their eyes," Otto described, adding, "She allowed them to just feel that moment, like, 'OK, it's gonna be all right. There is some hope here. We're gonna make it through this.'"

Otto's research into the dogs who served at ground zero over the past two decades has revealed positive outcomes for their health and welfare. The study found that the search-and-rescue dogs experienced minimal health risks and, in many cases, outlived the average lifespan for their breeds. The last known surviving search dog from 9/11 passed away in 2016.

"We feel like the dogs that are doing this kind of work, they're physically active, they're mentally engaged, they have this wonderful relationship with their handler," Otto explained. "We think that's really actually good for them and good for their health and welfare."

Furthermore, the presence of these canine partners appeared to benefit their handlers' mental well-being. Research indicated that first responders who had a search-and-rescue dog with them at ground zero were less likely to develop post-traumatic stress disorder compared to those who did not.

For Schnelle, whose dog Atlas died in 2008, the legacy of these dogs is their unwavering dedication. "Atlas was like me, he just did it. He never shut down... he just did his job," he said, reflecting on their service in unimaginable conditions.