A little after 1 p.m. on a wintry day in January 2011, Detective Stacy Galbraith approached a long, anonymous row of apartment buildings that spilled up a low hill in a Denver suburb. Snow covered the ground in patches. It was blustery, and biting cold. She was there to investigate a report of rape. Galbraith spotted the victim standing in the thin sunlight outside her ground floor apartment. She was young, dressed in a brown, full-length coat. She clutched a bag of her belongings in one hand. She looked calm, unflustered. Galbraith introduced herself. Police technicians were swarming the apartment. Galbraith suggested that she and the victim escape the icy gusts in a nearby unmarked patrol car. The woman told Galbraith she was 26 years old, an engineering student on winter break from a nearby college. She had been alone in her apartment the previous evening. After cooking green mung beans for dinner, she curled up in bed for a marathon of “Desperate Housewives” and “The Big Bang Theory” until drifting off. At around 8 a.m., she was jolted awake by a man who had jumped on her back, pinning her to the bed. He wore a black mask that seemed more like a scarf fastened tight around his face. He gripped a silver and black gun. “Don’t scream. Don’t call or I’ll shoot you,” he told her. He moved deliberately. He tied her hands loosely behind her. From a large black bag, he took out thigh-high stockings, clear plastic high heels with pink ribbons, lubrication, a box of moist towelettes and bottled water. Over the next four hours, he raped her repeatedly. He documented the assault with a digital camera and threatened to post the pictures online if she contacted the police. Afterward, he ordered her to brush her teeth and wash herself in the shower. By the time she exited the bathroom, he had gone. He had taken her sheets and bedding. She clearly remembered one physical detail about him: a dark mark on his left calf the size of an egg. Galbraith listened to the woman with a sense of alarm. The attack was so heinous; the attacker so practiced. There was no time to waste. Sitting close to her in the front seat of the car, Galbraith carefully brushed the woman’s face with long cotton swabs to collect any DNA traces that might remain. Then she drove her to St. Anthony North Hospital. The woman underwent a special forensic examination to collect more DNA evidence. Before she left with a nurse, the woman warned Galbraith, “I think he’s done this before.” Galbraith returned to the crime scene. A half-dozen officers and technicians were now at work. They were knocking on neighbors’ doors, snapping photographs in the apartment, digging through garbage bins, swabbing the walls, the windows, everywhere for DNA. In the snow, they found a trail of footprints leading to and from the back of the apartment through an empty field. They spraypainted the prints fluorescent orange to make them stand out, then took pictures. It was not much. But something. One officer suggested a bathroom break. “Just keep working!” Galbraith insisted. As she headed home that night, Galbraith’s mind raced. “Who is this guy?” she asked herself. “How am I going to find him?” Galbraith often volunteered to take rape cases. She was a wife, a mother. She was good at empathizing with the victims, who were overwhelmingly women. Most had been assaulted by a boyfriend, an old flame, or someone they had met at a club. Those investigations often boiled down to an issue of consent. Had the woman said “yes”? They were tough for cops and prosecutors. Juries were hesitant to throw someone in prison when it was one person’s word against another’s. Rapes by strangers were uncommon — about 13 percent of cases. But there was still the issue of the woman’s story. Was she telling the truth? Or fabricating a ruse to cover a sexual encounter gone wrong? In that way, rape cases were unlike most other crimes. The credibility of the victim was often on trial as much as the guilt of the accused. And on the long, fraught trail between crime and conviction, the first triers of fact were the cops. An investigating officer had to figure out if the victim was telling the truth. Galbraith had a simple rule: listen and verify. “A lot of times people say, ‘Believe your victim, believe your victim,’” Galbraith said. “But I don’t think that that’s the right standpoint. I think it’s listen to your victim. And then corroborate or refute based on how things go.” At home, her husband David had done the dishes and put the kids to bed. They sank down on separate couches in their living room. Galbraith recounted the day’s events. The attacker had been cunning, attempting to erase any traces of DNA from the scene. Before he left, he showed the student how he broke in through a sliding glass door. He suggested she put a dowel into the bottom track to keep out future intruders. The victim had described him as a “gentleman,” Galbraith said. “He’s going to be hard to find,” she thought. David Galbraith was used to such bleak stories. They were both cops, after all. He worked in Westminster, some 15 miles to the northeast. Golden and Westminster were middle class bedroom towns wedged between Denver’s downtown skyscrapers and the looming Rockies. This time, though, there was something different. As David listened, he realized that the details of the case were unsettlingly familiar. He told his wife to call his department first thing in the morning. “We have one just like that,” he said.