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In 1990, Merck discovered what she now refers to as “the dark side.” While she was a feline specialist at a veterinary practice in Atlanta, a couple turned up with a kitten that they claimed had injured itself under a kitchen cabinet. Merck’s examination determined otherwise: The kitten had been beaten. She called the police, and within a week, the couple was charged with animal cruelty.

Officers told her they’d never heard of a vet reporting suspected abuse before, which surprised her. “Vets are natural detectives,” she says. “We have to figure it out every time a patient comes in with clues from the animal, our diagnostics, the owner.”

Nevertheless, according to Merck, her boss at the practice was less than thrilled by her sleuthing. The law doesn’t require vets to involve themselves or the police in suspected animal-abuse cases, and doing so risks alienating some pet owners. Merck knew she had to quit. “There’s a moral aspect to being a vet that is more important than whether or not a bill is going to be paid,” she says. She set up her own veterinary practice.

A year later, a distraught woman showed up with a dog that had been stabbed by her boyfriend, Merck says. Once again, she alerted the authorities. But this time the behind-closed-doors scene was more complex. When the police questioned the couple, it emerged that the dog wasn’t the only victim of abuse in the household. The boyfriend was arrested for domestic violence against his partner.

That was Merck’s light-bulb moment—the moment the dark side became clear. “People who commit animal cruelty are usually criminals in other ways,” she realized.

Eager to determine just how strong such an association might be, Merck embarked on a 15-year quest to learn everything she could about forensics. On top of running her practice, she spent her spare time traveling around the country to participate in classes like “Death-Scene Investigations in the Natural Environment,” “Gunshot Reconstruction,” and “Death-Scene Checklist.” When she enrolled in a bloodstain-patterns course in New York, she was the only non–police officer there. Over time, she learned how to take DNA swabs, how to use a blue light to reveal whether bloodstains are from humans or other mammals, and how to photograph crime scenes.

By 2009, Merck was armed with the kind of arsenal of forensic knowledge one would normally only associate with law enforcement. She left her practice to set up an animal-forensics consultancy in Austin. In partnerships with the American Society for the Prevention of Cruelty to Animals (ASPCA) and the Animal Legal Defense Fund, she began persuading police officers and attorneys to rely on her unusual expertise. “It’s foreign to them, what the medical records, what the words, the abbreviations, the exam, or necropsy findings mean,” she says.