Should a dog’s sniff be enough to convict a person of murder?

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A version of this story appeared in Science, Vol 374, Issue 6565.Download PDF

Mark Redwine kept mementos of his youngest son in the cab of his truck. The boy’s name, Dylan, is tattooed on his left shoulder. He distributed missing person posters at truck stops. In November 2012, his 13-year-old son had arrived to spend Thanksgiving at Redwine’s cabin, near the San Juan National Forest in southwestern Colorado. Redwine told an FBI investigator that he left Dylan asleep on the couch when he went to town. When he returned, the boy was gone.

More than 6 months later, hikers found skeletal remains on a trail high in the desert-clear air, about 16 kilometers from Redwine’s cabin. Authorities confirmed the bones belonged to Dylan, but the local coroner could not determine cause of death. To go missing is not a crime, but Redwine’s ex-wife, Elaine Hall, and the couple’s oldest son came to suspect Redwine did something to make Dylan disappear. Redwine flatly denied any involvement, but police soon homed in on him. On 5 August 2013, the investigation took a dramatic turn when Carren Corcoran arrived in Durango, Colorado, with her German shepherd, Molly. At the time, Corcoran worked for the Madison, Wisconsin, police department. In her off hours, she ran Canine Search Solutions, a private firm that specialized in using dogs trained to sniff out cadavers to search for missing people. By her count, she had worked 265 cases in 30-odd years. Molly, or “Bitty,” as Corcoran called the dog, was unusually small for her breed. Molly had started out as “a timid little puppy that was given up to a rescue,” Corcoran later wrote, but under her tutelage the dog became “a legend in the field of human remains detection.”

Corcoran and Molly met local police at a La Plata County Sheriff ’s Office warehouse known as Area 51. Inside, Corcoran let Molly off leash. “Go find,” she said. Nose down, Molly sniffed at the concrete floor. Corcoran understood the dog’s behavior to mean the area was all clear—free of odors. Then she asked investigators to bring in the evidence, three paper packages containing clothes Mark Redwine had allegedly worn at the time of Dylan’s disappearance. Corcoran again commanded Molly to search. This time, her dog went up to the bags, gave a deep, audible sniff, looked Corcoran in the eye, and sat down next to two of them. Because Corcoran had trained Molly to sit when the dog detected the odor of human remains, those “indications” suggested the odor of a dead body lingered on jeans, a pair of sneakers, and a work shirt.

Next the team went to Redwine’s cabin, which was unoccupied. Molly sat down in a nearby stream and indicated at five other locations outside the home, according to Corcoran. Then she saw her dog looking up at a window. An investigator climbed into the house and opened the door for Corcoran and Molly. Inside, the dog made its indication—a sit—near a wood stove, in a bedroom, on a brown sofa, by a washing machine, and elsewhere. In February 2014, police called Corcoran back and had Molly sniff Redwine’s Dodge pickup truck. Again, Corcoran said, her dog detected the odor of human remains, more than a year after the boy’s disappearance.

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It’s not enough to say I have this amazing expert. … You have to explain exactly what their method is.

  • Binyamin Blum
  • University of California Hastings College of the Law

Nearly 4 years later, Redwine, driving a semitrailer, pulled into a parking lot in Bellingham, Washington. Police officers approached and arrested him on suspicion of killing his youngest son. In a body camera video of the encounter, released to a local TV station, Redwine looks disheveled and confused. “If you guys can, can you please explain to me what’s going on?” State prosecutors in Colorado persuaded a grand jury to indict Redwine on some of the most serious charges the state could pursue: child abuse and second-degree murder. His trial began in July and the prosecution’s narrative was built, in no small part, on commonly held assumptions about the behavior of a scent-detecting dog.

Dogs have been celebrated since antiquity for their ability to sniff a particular odor and lead humans to its source. But the domesticated canine’s transformation into crime-fighting companion emerged much more recently, as U.S. police launched K-9 training programs and a thriving cottage industry of private firms, which often aid law enforcement, emerged. Today, police use dogs to track fugitives and search for missing persons, obtain probable cause (that is, legal justification to get a search warrant), and find substances, particularly illegal drugs. In what are known as scent lineups, agencies use trained canines to match evidence collected at a crime scene to the scent of a suspect or body. Increasingly, testimony from dog handlers has also served as direct evidence of guilt—accepted in lieu of an actual corpse, drug stash, or other physical evidence of a crime.

Yet critics worry that the criminal legal system has embraced a technique profoundly lacking in scientific validation. Dog-sniff evidence has led to wrongful convictions, and studies show human biases skew animal behavior. Almost no published research indicates just what dogs detect or how they do it. Defendants and their lawyers can’t cross-examine a dog, which means the accused cannot scrutinize the evidence or readily confront their accusers, a right enshrined in the U.S. Constitution.

“It’s not enough to say I have this amazing expert with an incredible nose who can distinguish between scents,” says Binyamin Blum, an evidence scholar at the University of California (UC) Hastings College of the Law, who contends such testimony short-circuits the safeguards in place to discriminate between junk science and real science. “You have to explain exactly what their method is.”

In 1990, about the time Corcoran joined the police force in Madison, she read a book, So That Others May Live: Caroline Hebard & Her Search-and-Rescue Dogs, describing one woman’s passion for finding missing people. She was intrigued, but her bosses were not initially interested in deploying K-9s. So Corcoran began to train German shepherds on her own time. Between 1999 and 2001, she went to five seminars hosted by Canine Solutions Inc. (CSI) and taught by Sandra Anderson, a trainer with an uncanny knack for turning up evidence where others found nothing. (Sometimes no evidence existed: The FBI accused Anderson of planting bones and other remains for her dogs to find, and she went to prison in 2004.) In 2005, Corcoran founded Canine Search Solutions, attracting volunteers who shared a love for dogs, the outdoors, and in the words of one colleague, “solving mysteries and bringing closure to families.”

Corcoran’s biggest break came in 2004 when she cracked a cold case. A Wisconsin woman had gone missing in 1976. Nearly 4 decades later, Corcoran’s dog Cleo indicated in 14 locations where the woman’s ex-husband, the chief suspect, allegedly killed her and then tried to hide the body. No remains were found, and a judge barred Corcoran from testifying in the trial. Nonetheless, police said the suspect confessed and admitted to dumping the body at the locations Cleo flagged. Soon, other prosecutors were asking Corcoran to testify in trials. In court transcripts, she described dogs as “terrible poker players,” incapable of deceit. As for Molly, she told one court last year, “I would put my house up against her indications.”

Carren Corcoran on the witness stand, with a photo of a German shepherd on a screen behind her.
Carren Corcoran Gummin’s testimony about the “indications” provided by her German shepherd, Molly, helped convict Mark Redwine of murder this summer. Jerry McBride/The Durango Herald

Her career arc mirrors a broader shift in the United States. The number of police dogs swelled from 2000 in 1989 to as many as 50,000 in 2010. After the 2013 Supreme Court ruling in Florida v. Harris, which found drug-detection dogs reliable as long as the dog-and-handler team was properly trained and certified, judges increasingly allowed expert witnesses to testify about dog-sniff evidence.

As with the 2004 cold case, the charges against Redwine hinged on Molly’s ability to detect the odor of dead bodies where no human remains could be found. “The best way I can describe it is someone pops popcorn in your house,” Corcoran explained to one Wisconsin jury. “You come in. You can smell the popcorn that’s been popped, but there is no popcorn left.” Suspects could hide a body but, as she saw it, no one could outrun a dog’s nose.

The notion has plausible roots. Behind a dog’s leathery, wet nose lies a cavernous labyrinth of scroll-shaped chambers called ethmoturbinates lined with some 200 million olfactory receptors, encoded by an estimated 2.5 times as many genes as in humans. In recent years, researchers studying canine cognition have shown pet dogs can sniff out minute quantities of odorants, such as the odor of their owner’s T-shirt after it has been worn.

In the best known study on what is sometimes called residual odor—one that Corcoran has cited in court—German researchers placed carpet squares underneath two recently deceased corpses and then presented those squares to three trained Malinois. The 2007 study showed the dogs accurately distinguished those squares from negative controls, though critics contend the findings were muddied by possible contamination and poor experimental design.

But what odors would be left 7 months or more after Dylan’s alleged murder? Although forensic anthropologists have identified hundreds of compounds associated with cadavers, little evidence suggests all dead bodies immediately start to give off a standard, identifiable odor signature. John McGann, a neuroscientist at Rutgers University, New Brunswick, who wrote an influential review paper in Science arguing that poor human olfaction is a myth, says, “Even in cases where you say, ‘OK, a dog can follow a scent trail through the woods and find a person.’ Yes, they can. But what are they smelling? We don’t know. Right? We just literally don’t really know what exactly they’re smelling.”

In the decade leading up to Mark Redwine’s trial, Corcoran had amassed an impeccable track record. Prosecutors had called her to testify in six criminal cases. Each time judges allowed her testimony to be introduced before a jury, the suspects were found guilty, and unexplained deaths became criminally punishable homicides.

On 27 June 2019, almost 7 years after Dylan disappeared, Dana Delger and M. Chris Fabricant, attorneys from the Innocence Project’s strategic litigation department, arrived at the La Plata County Court in Durango. Their goal was to challenge Corcoran’s technique and persuade Judge Jeffrey Wilson not to admit the dog-sniff evidence.

Fabricant, hired by the Innocence Project in 2012, had assembled a portfolio of questionable forensic techniques. By the time of Mark Redwine’s trial, dog-sniff evidence was high on that list. For one thing, dogs were prone to error. In data from Australia, police recorded 10,211 alerts made by drug-sniffing dogs; 74% of the hits turned out to be false alarms. But Fabricant was especially concerned about instances where canines provided direct evidence.

In one case, a Michigan man named D’Andre Lane told police his daughter had been kidnapped during a carjacking. But after an expert claimed his dogs detected the odor of decomposing remains in Lane’s car, a jury found him guilty of murder. In another, an Arizona woman was sentenced to life in prison for murdering her daughter and throwing the body in the trash. Prosecutors called two handlers who claimed their dogs detected the scent of death in a dumpster. A body was never found in either case, but the testimony helped seal the suspect’s fate.

Those cases suggested a larger pattern. A dog helped turn the loose threads in the prosecution’s case into solid narrative rope. “The cadaver dog is bridging the gap between pretty plausible suspect and guilty beyond a reasonable doubt,” Delger says, emphasizing that she could not comment on any particular case.

Follow the evidence?

Molly, a dog trained to find cadavers, stopped at multiple locations near Mark Redwine’s cabin in Colorado, supposedly after detecting the odor of human remains. No remains were found at any of the locations, and the GPS coordinates do not appear to match where investigators previously collected physical evidence.

A map showing where evidence was found and where the dog indicated.
(Graphic) N. Desai/Science; (Data) La Plata County Sheriff’s Office; Carren Corcoran; People of the State of Colorado v. Mark Redwine (2017)

What happened in trials without any testimony about dogs was also telling. In late 2019, Fabricant and Delger helped a Wisconsin attorney representing a man accused of killing his ex-girlfriend and dumping her body in a lake. Corcoran and a colleague claimed dogs detected the odor of human remains inside the suspect’s home, but the defense convinced a judge that the testimony did not meet the legal standard for scientific evidence. The suspect was found not guilty—ending Corcoran’s winning streak.

No comprehensive database exists, but according to the National Registry of Exonerations, a project hosted by the University of Michigan Law School, at least 17 innocent people have been freed after dog-sniff evidence erroneously sent them to prison. Other convictions have been upheld—even after errors came to light. In 2000, Anderson, who trained Corcoran at CSI, helped persuade a jury in Madison to convict a Tanzanian immigrant accused of murdering his cousin. In court, Anderson paraded her Doberman pinscher in front of a jury, claiming the dog sniffed blood inside the suspect’s home. Even though Anderson was later jailed for faking evidence, a judge upheld the life sentence, saying the discredited handler’s testimony was “harmless.” In a letter from prison, Peter Kupaza, the man convicted of the murder, told Science he “did not commit this crime [and] that is 100% fact.”

By 2019, when Fabricant and Delger submitted motions in Colorado, they had honed their argument. The Innocence Project attorneys did not question whether dogs could detect odors imperceptible to humans. Rather, they argued, “What is at issue is what has never been proven with any degree of scientific reliability: the ability of a dog to detect the residual scent of a particular object, including human remains, at a specific location days, weeks, months, or even more than a year after that object was removed.” A sniff could be used to corroborate, but they argued a dog’s indications alone should not be used to prove a person’s guilt.

Fabricant and Delger argued that a dog’s behavior may reflect a handler’s expectations, pointing to a 2011 study in Animal Cognition by Lisa Lit, then at UC Davis. Lit found handlers “cued” dogs into making false indications. In one test, Lit showed up each morning with evidence bags containing cannabis and gunpowder, explaining to 18 teams that those target scents might be present inside a church. No target odors were present, and yet dogs positively indicated 85% of the time, handlers said, suggesting the dogs served as loyal companions first and objective scent detectors second.

Similar issues came to light more recently in letters criticizing a 2018 study published in Forensic Science International. The experiment was designed to test the long-standing belief that dogs could “mantrail” unfamiliar suspects by their odor. In the study, dogs correctly trailed people 82% of the time. But earlier this year, several critics pointed out methodological shortcomings stemming from handler and experimenter bias, and the journal’s editors added a cautionary “expression of concern.”

Such biases may be impossible for dog handlers to avoid, Delger and Fabricant argued in their motion to exclude handler testimony in the Redwine case. A dog is “a companion animal with a vested interest in pleasing its master,” they argued. Moreover, dogs are astute students of deliberate human signals and unintentional cues, paying close attention to people’s faces, particularly our eyes.

A German shepherd, followed by two police officers.
The number of police dogs in the United States grew from 2000 in 1989 to as many as 50,000 in 2010.Africa Studio/Shutterstock

Corcoran, who went by Carren Corcoran Gummin during the Redwine trial, stands by her work. In an interview with Science, she compared her critics to flat Earthers, saying she had “spent the last 20 years being criticized by people who do not understand how cadaver dogs work.” She saw her work as one tool to extract information where there otherwise was nothing. “There’s so many missing people that need to be found. You know, it’s just sad. And that’s really what it’s all about: If we can recover somebody, so their family can know the ending. That’s really what it should be about.”

At the pretrial hearing in Durango, Delger, who took the lead in cross-examining Corcoran, asked whether dogs could reliably indicate in areas where no remains turned up. She asked the handler: “So there might be odor and you haven’t recovered remains?”

“Correct,” Corcoran said.

“But you might also have a false positive?”

“That’s absolutely positive—or possible,” Corcoran said.

Delger asked Corcoran whether Molly’s indications on Redwine’s property could be independently confirmed. “There’s no test other than the dog that can tell us whether odors are there?”

“The testing that we conduct is simply: ‘Is there odor there or not?’” Corcoran said. Molly indicated, she said, which put the odor of a dead body, at some point in time, in contact with the suspect’s property.

Wilson allowed the testimony, citing an earlier ruling in Colorado that found that although dog tracking was not scientific, the criminal legal system accepted the technique as “common knowledge.” The judge also noted that Corcoran would be a star prosecution witness.

On 6 July, the 11th day of Redwine’s trial, the prosecution called Corcoran to the stand. She described how Molly made more than 40 trained final indications, sitting near Redwine’s clothes in Area 51, in and around the defendant’s house, and in 12 places near where bones were found. That all suggested, as Corcoran told jurors, “She detected the odor of human remains.”

Redwine’s defense team raised the lack of a scientific basis for dog-sniff evidence and the possibility that dogs reflect explicit prejudices, implicit biases, or simply the very human desire to fill in gaps in our perception. But research suggests many jurors simply don’t see it that way. In 2019, Lit teamed up with Itiel Dror, a cognitive neuroscientist from London who studies expert decision-making. They asked 554 mock jurors to read a story about a dog detecting the odor of drugs on a suspect’s car even though a later search turned up cash but no evidence of illegal activity. Lit says the scenario was meant to sound preposterous. Nonetheless, two-thirds of jurors surveyed said they would have found the suspect guilty.

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The cadaver dog is bridging the gap between pretty plausible suspect and guilty beyond a reasonable doubt.

  • Dana Delger
  • Innocence Project

None of the three major accrediting organizations that certify cadaver dogs tests their ability to detect residual odor, and little published research shows dogs are capable of the feat. “I don’t even know how you would even attempt to do that,” says one of Corcoran’s mentors, an official at an accrediting organization. Dror, by contrast, says a rigorous experiment would not be hard. “It’s not rocket science. It’s very simple.” He suggests putting out cadavers for 1 week, removing the remains, and then asking handlers—who knew nothing about the experimental setup—to conduct a search.

Still, Dror says he understands why people put their faith in untested claims. People want justice. Humans seek cognitive closure. “You sleep better at night—the detectives and the juror do—if they find who did it and do not leave an unsolved crime,” Dror told Science. “Here the dog comes and tells them.”

In the days before Corcoran’s testimony, the evidence that Redwine murdered his son remained overwhelmingly circumstantial. The prosecution called Dylan’s girlfriend, who tearfully remembered him saying he did not want to visit his father. Dylan’s older brother hinted at a possible motive, saying Dylan was going to confront his father about photos allegedly showing Redwine eating what appeared to be feces out of a diaper. (As a New York Post headline put it, “‘Compromising’ fetish pics led Colorado dad to kill teen son, prosecutors claim.”) The boy’s skull contained apparent fractures, and though the possibility that cattle or scavenging wildlife damaged the bones after death was hard to rule out, the prosecution called on an expert who said a sharp object was responsible. No weapon was ever found.

The only direct evidence putting a dead body inside the suspect’s home and his pickup came from Corcoran’s testimony. She had trouble remembering details from a search 8 years earlier, and she broke down in tears when prosecutors displayed a photo of Molly, who died in 2018, on a screen behind the stand. None of the GPS coordinates Corcoran recorded on the mountain matched the location of any skeletal remains (see graphic, above). But that did not necessarily prove Molly wrong, she said, because odor could potentially travel down the mountain in streams and rivers.

As the trial wound down, the defense called another expert, James Ha, a researcher at the University of Washington, Seattle, who studies applied animal behavior but—as prosecutors got him to admit—has little firsthand experience with cadaver dogs. Ha questioned whether odors could linger for more than 48 hours because they are volatile and likely to dissipate. Given the lack of published research and the difficulty in ruling out false positives, he said, scientists generally did not accept residual odor as a valid concept. Ha argued that without tangible evidence to validate an indication, a dog alone could not yield any information. “We don’t know how to get inside a dog’s mind,” Ha told the jury. “They’re not machines.”

Redwine chose not to testify. Six months after his son’s disappearance, he wrote in a statement to local media, “I absolutely do not know where Dylan is.” His attorneys tried to paint him as a loving father who must have felt guilty for losing his son on his watch. In a case built on circumstantial evidence, the trial pitted his claims of innocence against a dog’s telling indications. Twelve people were asked to decide whose story to believe.

In late July, after more than 6 hours of deliberation, the jury found Redwine guilty—an outcome that cemented Corcoran’s reputation for convincing jurors that dogs just know what the rest of us could not.

This story was supported by a grant from the Massachusetts Institute of Technology Knight Science Journalism fellowship.