Money stood aside as the photographer took pictures of the tire tread in the mud on the corner of South and Bussey.

“I think we can get a decent mold,” Eunice Curran said.

“Good. I’ll see you back on the hill.” He turned and followed the asphalt path, partly hidden in shadow, toward the opening ahead. The area looked so different at night. He remembered coming here on one of his first dates with Leslie. A warm spring day. It was Lilac Day, and Leslie thought it would be nice to go for a walk and have some bread and cheese outdoors near the little brook that ran through the woods.

Like his two unidentified victims on the hill.

That was a long time ago. Before he’d seen so much death. He and Leslie had stopped at the lilacs as they made their way through the maze of paths that wound through the trees. Peter’s Hill and the rest of the Arboretum were maintained by Harvard University, she’d explained to him. The best kept park in the city, she’d said. The most beautiful jewel in the…

Mooney stopped. He was alone, not quite at the path at the base of the hill where most of the other units were gathering. One by one, and in order, he ticked off the murder sites. The Fens. The Riverway. Olmsted Park. Franklin Park. And now Peter’s Hill, the Arnold Arboretum. It made perfect sense.

He picked up his pace. At the base of the hill, he stepped off the path and cut across the grass toward the scene the killer had left for them. He spotted Alves walking Connie through it.

When Mooney caught up with them, Alves said, “Connie doesn’t think the murders have anything to do with prom night. Thinks he dressed them up as newlyweds for their picnic in the park.”

“Interesting. ’Cause I don’t think this has anything to do with a picnic in the park.” Mooney waved his hand at the victims. “It’s more like a picnic on the Emerald Necklace.”

“I don’t get it,” Alves said.

“He’s not familiar with Boston’s history,” he said to Connie. “This minute, we’re standing on Peter’s Hill, which is a part of the Arnold Arboretum. Which is-”

“One of the jewels in Olmsted’s Emerald Necklace,” Connie interrupted.

Mooney nodded, then turned to Alves. “You’ve never heard of Frederick Law Olmsted, have you?”

Connie began, “Olmsted designed half of Central Park in New York City. Then he did the system of parks in Boston that runs from the Common to Franklin Park. Each one is a ‘jewel’ in what he called the Emerald Necklace. What kind of Bostonian are you?”

“I’m from Jamaica Plain,” Alves said.

“Most of the Necklace is in J.P.,” Mooney continued. “The Arborway, the Arboretum, Jamaica Pond, Franklin Park-”

“Got it. I’ll study up on my history of the Boston Parks tomorrow. How does this tie in?” Alves asked Mooney.

“The Boston Common and the Public Garden are the first two jewels in the necklace. Then you have the Commonwealth Mall, the grassy area that runs down the middle of Comm Ave. That leads right into the Back Bay Fens where Kelly Adams and Eric Flowers were found. Then you have the Riverway, which leads into Olmsted Park and the Jamaica Pond.”

“So the killer’s taking us on a tour of the Emerald Necklace,” Connie said. “But why?”

“Don’t know yet. Maybe Adams’s necklace gave him the idea to take us on a tour of his Emerald Necklace. Maybe he works for the Parks Department, a laborer, a supervisor.” Mooney paused. “Or a park ranger. Someone with a badge who might be able to gain your trust.”

Mooney studied the two men in front of him on the dark hill. One of them was a student of Boston’s history, the other was not. The killer was someone with knowledge beyond knowing that kids from Dorchester hated kids from Southie, and that kids from Southie hated kids from Charlestown. The killer was someone who understood Boston. Here, all along, they’d been thinking that the killer was giving them clues-the Tai-ji stamps and the fortunes. That was crap. The real clues were much more subtle. The killer was challenging them on a level he didn’t usually find in criminals.

Connie had seen something at Peter’s Hill that he hadn’t mentioned to Alves. He was irritated that Alves had kept the Tai-ji from him. Alves had never held back anything before. Maybe Alves was following Mooney’s orders to keep the symbol a secret. Maybe Alves didn’t trust him. For whatever reason, things had changed.

But Connie had the details now. He stretched out on the couch in his basement. The one place he could really focus. Here he could block out distractions and think, and now he was running possible scenarios through his head.

The Tai-ji symbol might be the key to everything. It represented the Yin and the Yang, symbols of the opposite forces of nature, in balance, continually changing. But what did the symbol mean to the killer? Was it an obsession, however misguided, with Chinese culture and philosophy? Or was he trying to give them a false lead? Either way, it would reveal something about the killer.

Interesting thought. What if it was just by chance that the first female victim had a Tai-ji tattooed on the back of her neck? Now, maybe, he was copying the symbol to make each murder look the same.

He believed that the first murders were the most important. The murders of Adams and Flowers were unorganized, spontaneous, unplanned. Something provoked the killer to strike. Stressors. A lost job, a fight with a girlfriend, sure, but more likely something like a surge of electricity scouring through the circuitry of the body till there was no choice but to act.

Connie needed to bring himself back to that time, to visualize things as they were ten years earlier. The murders had all occurred in the summer months, when Connie was home from college in Arizona, between his junior and senior years.

Connie remembered where he was and what he was doing that summer, but he needed to put himself back into the climate in the city. He couldn’t just think back on the time, he had to relive it. Then he could turn his focus on the murders and put himself in the place of the killer.

That was something that Alves and Mooney didn’t understand. They were good at processing crime scenes and pursuing leads doggedly, but they had no idea how to think like a killer.

Connie was good at that.

Sleep was a careful driver, not too fast and certainly not too slow. Just a couple of miles over the speed limit so as not to draw attention to himself. He didn’t want anyone to notice him, especially the police. His headlights, taillights and signals were all working properly. He checked them each time before he went out.

He would have liked to have gotten closer to the investigation on Peter’s Hill, but he knew where to draw the line. Push too hard and people start asking questions. He had come close enough to get a taste of the investigation, to smell the scent of the grass on a cool autumn evening, to imagine himself back on that hill with the young lovers. They must have been marvelous under those brilliant lights.

He was sure no one had noticed him, tangled in the stalled traffic. And if they did bother to take a look, that’s where his little disguise came in handy.

Now he needed to get back to work, check on his next subjects. He knew their hangouts. It was amazing how many couples were out there every night. He was sure their parents didn’t know what they were up to, drinking and partying. Or maybe their parents didn’t care.

The not caring, the indifference, that’s what made it so easy for Sleep and his brother Death to enter their lives.