(CNN) The extensive search for four missing young men on a sprawling property in Pennsylvania has involved about 50 law enforcement investigators, including local and state police and the FBI.
But the big break in the case -- a 12-and-a-half-foot deep grave with the body of one of the missing men and other human remains -- came from team members of a different species.
"We had cadaver dogs," Bucks County District Attorney Matt Weintraub said, shaking his head in amazement.
"I don't understand the science behind it, but those dogs could smell those poor boys 12-and-a-half feet below ground."
Those cadaver dogs were central in locating the body of Dean Finocchiaro, 19, one of four missing men in Bucks County, he said. The other three men, Jimi Patrick, 19, Mark Sturgis, 22, and Thomas Meo, 21, have not yet been found. Additional human remains were found in the grave but have not been identified.
The science of cadaver dogs is a fascinating combination of biology and single-minded training, said Cat Warren, the author of "What the Dog Knows: Scent, Science, and the Amazing Ways Dogs Perceive the World."
"I'm a real believer in the purity of these dogs when they're properly trained," Warren said. "They do have good noses. They are able to distinguish that faint scent -- that human scent."
Dogs of all skill sets -- such as smelling for narcotics or for explosives -- are used in different roles as part of law enforcement teams. Cadaver dogs are trained to identify a scent specific to dead human bodies and then notify their trainers.
These dogs are often used in death investigations, such as in the Casey Anthony case, or during the search for survivors after the crash of Malaysia Airlines Flight 17 in Ukraine.
Training goes a long way, but dogs are genetically better than other animals at sniffing out specific scents, said Dave Pappalardo, owner of K9 Unlimited, which trains dogs for law enforcement and civilians.
"The dog sees the world through its nose first, before its eyes," he said. "Just like we look at things first, they smell things first."
Imagine, Pappalardo said, a dog and a human walk into a house with soup cooking. The human smells soup, full stop. The dog, though, smells every ingredient in the pot individually and clearly.
"That's why they're so good at their job, they can discriminate odors individually," he said.
One of those distinct odors is human bodies, and the dogs are trained to recognize that specific smell and brush aside others.
"There is a lot of crossover with dead animals and dead humans, but humans have enough distinct things about them that cadaver dogs can tell the difference," Warren said.
The dogs hone those skills with training that uses real donated human body parts, including placenta, teeth, bones and human fat, she said. At some research facilities, including Western Carolina University, dogs are trained using actual entire human bodies, Warren said.
At a sprawling and busy investigation like the one in Bucks County, the ability of a dog to narrow down the search to one area can be hugely valuable.
"Dogs are smelling all sorts of other things when on a farm or construction site," Warren said, "but there's only one set of odors that the dog is going to get a reward for if it's a cadaver dog, and that's the scent of human remains."
On Tuesday, Weintraub, the district attorney, said that local and state police and the FBI were in an "all hands on deck" investigation to find the missing men.
All paws, too.