In the aftermath of California’s recent forest fires, cadaver dogs sniffed beyond burning homes, charred forest, and even other dead animals to pick up the unique scent of human victims. A new study reveals how they might have done it: Decomposing humans seem to release a singular chemical cocktail, one that scientists might be able to use to better train cadaver dogs and even develop machines that could do the same job.

Researchers have been trying to understand the “smell of death” for more than a decade. Two papers kicked off the effort in 2004, one from a Tennessee research station called the Body Farm that evaluated gases released late in decomposition, and the other from Greece, which looked at the early stages of the process. Since then, the list of carbon-based, or organic, compounds given off during decay has grown quite long, but there have been conflicting reports about which ones are emitted only by humans.

In 2010 the Belgian Disaster Victim Identification Team asked analytical chemist Eva Cuypers and her forensic toxicology lab at the University of Leuven in Belgium for help finding the best way to train cadaver dogs to pick out human scents. Cuypers’s graduate student Elien Rosier started by putting tissue samples and organs from six autopsied corpses in jars in a lab closet. The jars’ screw caps, which let in some air, had stoppered holes that allowed her to periodically take samples of the gases building up inside. She set up other jars with pig, mouce, mole, rabbit, turtle, frog, sturgeon, or bird remains. Pig remains in particular have often been used in past decomposition studies because of their similarities to human bodies (which are often hard to come by): They have the same microbes in their guts, the same percentage of body fat, and similar hair as people. But it was not clear whether the decomposition process was the same because the two species had never been studied under identical conditions.

Rosier identified the compounds in the collected gases and compared them among species. Over 6 months, she found 452 organic compounds. At first, sulfur-containing compounds seemed to distinguish the different species, but they were not unique to humans or even present in all humans. Further, they disappeared over time. Compounds called esters, a big component of animal fat, looked more promising. Ultimately, eight compounds distinguished pig and human remains from those of other animals, and five esters separated pigs from humans, the researchers report this month in PLOS ONE. “The mixture of [these] compounds might be used in the future to more specifically train cadaver dogs,” Cuypers says.

The finding is important because “so far there [hasn’t been] any study based on monitoring human and pig carcasses under exactly the same conditions,” says Agapios Agapiou, an analytical chemist at the University of Cyprus in Nicosia who was not involved with the work. “But there are still many steps before creating a synthetic substance to train cadaver dogs.”

“I don’t think there’s one specific thing that says it’s human,” adds John Sagebiel, an analytical chemist at the University of Nevada, Reno, who was also uninvolved in the study. He says relying solely on analytical chemistry to determine what cadaver dogs are sniffing out is too limited. It would be better to work with the dogs themselves to figure that out, he adds.

And Arpad Vass, who is associated with the University of Tennessee, Knoxville, Body Farm and has compiled a list of compounds released by decomposing humans, points out that the use of specific tissues—versus a whole body—and the isolation of those tissues in jars means the researchers are looking at just a subset of the bacteria and other environmental factors that influence decomposition.

Cuypers says her team plans to address some of these shortfalls in future studies. “The next step in our research is to see whether the same compounds are found in buried, full decomposing bodies in the field and to see whether dogs trained on the mixture respond more specific[ally] to human decomposing bodies.” If this cocktail passes muster, the find could pave the way for developing an electronic nose that can do what dogs do, she adds.