(CNN) For K-9 handler Wilma Melville and her dog Murphy, the aftermath of the 1995 Oklahoma City bombing was a frantic blur: the choking dust, the mountains of debris and the anguish of knowing people buried alive were praying to be found before time ran out.

"I was definitely impacted. It was a life-changing experience," Melville told CNN.

She returned to her Ojai, California, home physically and mentally drained -- and frustrated that there had been so few search dogs available to help.

"There were approximately 15 FEMA-certified search and rescue teams at that time," she recalled.

Melville resolved to find and train more.

People and dogs

Before she could find four-legged rescuers, Melville searched for human volunteers from the dog world and the disaster management fields.

"It was her passion and that vision that led her to recruiting some of the best volunteers that we've ever seen," said Denise Sanders, spokeswoman for Melville's organization, The National Disaster Search Dog Foundation.

In 1996, Melville established an ambitious mission for her non-profit: to train shelter dogs and partner them with first responders. Then she set out to find dogs.

"I would simply present myself to a shelter and tell them what I needed. And in most cases, those folks were willing to help," Melville said. "It was a surprise to both the shelter and myself because I was developing 'What type of dog am I looking for?' 'Which dogs will succeed?'"

With help from a police K-9 trainer, Melville learned that the dogs she needed were the ones most difficult to place in a normal family home.

"The exact same behavior that we look for in these dogs are usually the ones that land them on the unadoptable or euthanasia list," Sanders told CNN. "They're highly energetic. They're super-focused, laser-focused."

A group of candidates at the Search Dog Foundation's training center.

The foundation prefers Labrador retrievers, Belgian Malinois, golden retrievers, German shepherds or mixes of those breeds.

"Next, that dog has to have a complete medical and is brought to our national training center in Santa Paula, California," Melville said.

Through realistic training, the candidates learn to canvas debris fields, keep their footing in rubble piles and alert handlers to live human scents.

When dogs get injured or fail to show promise as rescuers, they do not get returned to the shelters. "We are happy to find them a different career if they still want to work and show that they need to work," Sanders said. "Or we find them a loving home if they prefer to be with a family."

But most dogs make the cut and meet their permanent handlers after six to nine months of training.

"The handler has to come to trust that dog as the dog has come to trust the handler," Melville said. "That and more can be accomplished at the national training center."

A search dog trains on the rubble at the Search Dog Foundation's training center.

Countless lives saved

Over the past 22 years, the National Disaster Search Dog Foundation and its alumni have saved lives in nearly 170 disasters, from Hurricanes Katrina, Harvey and Irma to the Montecito mudslides.

A search dog looks for victims in the Montecito mudslides.

Thirteen graduates worked the World Trade Center site following the 2001 attacks. They've also rescued disaster victims in Nepal, Haiti and Japan.

It is difficult to count the total number of people -- or K-9 rescuers -- who owe their lives to the National Disaster Search Dog Foundation. But just about all of the dogs eventually move on to well earned retirements.

"We replace that dog if anywhere between their nine to ten years, the dog cannot any longer do the job," Melville said. "That's a commitment that has been there from the start, and I'm happy to say exists today. To see the growth, the stability of the foundation, it's heartwarming."