She knew she didn’t need to spell out the rest of the argument to Max. If the blood sample belonged to William Summer’s imagined coconspirator, it meant that the accomplice had somehow left behind DNA evidence at only one victim’s house, and only with one small spot of blood, even though Summer himself had been unconcerned about leaving behind his own bodily fluids.

Ellie realized that she had developed a classic case of tunnel vision. When she first heard from Lindsay Kelly, they had reinforced each other’s suspicions, allowing them both to see what they had wanted to see. When Steve and Jess tried to expose the holes in the theory, she had dug in her heels, pushing herself further into the tunnel.

If only her first phone call had been to Max, who had never attempted to steer her in either direction. She said as much to Max.

“We had a training last year at the office,” he said. “A Hofstra law professor came in to talk about how mental shortcuts can lead to wrongful convictions. Anyway, she said one way to prevent tunnel vision is to force yourself to do the opposite. You heard about a DNA hit from the College Hill Strangler samples in the database and naturally assumed there was a connection. But think of all the other explanations for a blood sample from that house turning up now in East Hampton.”

She pulled up what she could recall about Janice Beale while Max filled a large pot with water and turned on a burner. “She was single and lived alone. She had broken up with her boyfriend a few months before she was killed. It’s certainly possible he cut himself at some point.” In his confession, Summer described to the police how he had first spotted her buying toothpaste at Target and had selected her as a potential new “subject.” When he followed her home and saw that her house was adjacent to Edgemoor Park, he thought it was a sign that she was a perfect target. He could watch her from the park for hours, planning his attack. But then a man had arrived. She kissed him. A boyfriend. He shifted his focus, searching for another subject, but still continued his strolls through the park. After three months without a single sighting of a male visitor to Janice’s house, Summer decided that he was finally ready to make his move. He had joked that his only fear was that an overly protective parent might call the cops on the middle-aged man who kept showing up at the summer camp to stare across the park.

Then she saw it.