I forced myself to stare at the pictures. If I blinked, if I turned away, if I flinched, I might not be able to look back. Beside me, Dean was looking at the pictures, too. He scanned them, left to right, up and down, like he was taking inventory, like the bodies in these pictures hadn’t ever been people: somebody’s mother, somebody’s love.

“Seven bodies,” Agent Locke said. “Five killers. Three of these women were killed by the same man. The remaining four were the work of four different killers.” Agent Locke tapped lightly on the top of each photo, bringing my eyes from one to the next. “Different victims, different locations, different weapons. What’s significant? What’s not? As profilers, a large part of our job is identifying patterns. There are millions of unsolved cases out there. How do you know if the killer you’re tracking is responsible for any of them?”

I could never tell when Agent Locke was asking a rhetorical question and when she expected an answer. A few seconds of keeping my mouth shut told me that this was an instance of the first.

Agent Locke turned to Dean. “Care to explain to Cassie the difference between a killer’s MO and their signature?”

Dean tore his attention away from the photos and forced himself to look at me. Studying mutilated bodies was routine. Talking to me—apparently, that was hard.

“MO stands for modus operandi,” he said, and that’s as far as he got before he shifted his gaze from my face to a spot just over my left shoulder. “Mode of operation. It refers to the method used by the killer. Location, weapon, how they pick victims, how they subdue them—that’s a killer’s MO.”

He looked down at his hands, and I looked at them, too. His palms were calloused, his fingernails short and uneven. A thin white scar snaked its way from the base of his right thumb to the outside of his wrist.

“A killer’s MO can change,” Dean continued, and I tried to focus on his words instead of his scar. “An UNSUB might start off killing his victims quickly. He’s not sure he’ll be able to get away with it, but with time and experience, a lot of UNSUBs develop ways to savor the kill. Some killers escalate—taking more chances, spacing their kills closer together.”

Dean closed his eyes for a split second before opening them again. “Anything about an UNSUB’s MO is subject to change, so while it can be informative to track the MO, it’s not exactly bulletproof.” Dean fingered the closest picture again. “That’s where their signature comes in.”

Agent Locke took up the slack in the explanation. “An UNSUB’s MO includes all of the elements necessary to commit a crime and evade capture. As a killer, you have to select a victim, you have to have a means of executing the crime unnoticed, you have to have either physical prowess or some kind of weapon to kill them with. You have to dispose of the body in some way.”

Agent Locke pointed to the picture that had captured Dean’s attention.

“But after you stab someone in the back, you don’t have to roll them over and pose their arms, palms up at their sides.” She stopped pointing, but kept talking—about other killers, other things that she’d seen in her work with the FBI. “You don’t have to kiss their foreheads or cut off their lips or leave a piece of origami next to the body.”

Agent Locke’s expression was serious, but nowhere near as detached as Dean’s. She’d been doing this job for a while, but it still got to her—the way it would probably always get to me. “Collectively, we refer to these extra actions—and what they tell us about the UNSUB—as a signature. An UNSUB’s signature tells us something about his or her underlying psychology: fantasies, deep-seated needs, emotions.”

Dean looked down at his hands. “Those needs, those fantasies, those emotions,” he said, “they don’t change. A killer can switch weapons, they can start killing on a quicker schedule, they can change venues, they can start targeting a different class of victims—but their signature stays the same.”

I turned my attention back to the pictures. Three of the women had been stabbed: two in back alleys, one in her own kitchen. The woman in the kitchen had fought; from the looks of the pictures, the other two had never had a chance.

“These two,” I said, pulling out the first two stabbing pictures. “The killer surprised them. You said the UNSUB stabbed this one from behind.” I indicated the girl on the left. “After she was dead—or close enough to it that she couldn’t put up much of a fight—he turned her over. So she could see him.”

This was what Agent Locke was talking about when she used the phrase deep-seated need. The killer had attacked this girl from behind, but it was important to him—for whatever reason—that she see his face and that he see hers.

“Don’t say he,” Dean said. He shifted, and suddenly, I could feel the heat from his body. “Say you, Cassie. Or say I.”

“Fine,” I said. I stopped talking about the killer—and started talking to him. “You want them to see you. You want to stand over them. And as they lie there dying, or maybe even after they’re dead, you can’t help but touch them. You straighten their clothes. You lay their arms out to the side.” I stared at the picture of the girl he’d attacked from behind, and something else struck me about it. “You think they’re beautiful, but girls like that, women like that, they never even see you.” I paused. “So you make them see you.”

I looked at the next picture: another woman, stabbed and found dead on the pavement. Like the first, she’d been chosen for convenience. But according to the notes on the picture, she hadn’t been stabbed from behind.

“It wasn’t enough,” I said. “Turning her over after she died, it wasn’t enough. So you took the next one from the front.”

Like the first victim, this one had been laid carefully on her back, her hair fanned around her face in an unnatural halo. Without even thinking about it, I took the third picture on the top row—a gunshot victim who’d died running—and set it aside. That wasn’t the work of the same UNSUB. It was quick and clean, and there wasn’t a whiff of desire about it.

Turning my attention to the bottom row of pictures, I scanned them, trying to keep my emotions in check the way Dean did. One of these four women had been killed by the same UNSUB as the first two. The easy answer—and the wrong one—would have been the third stabbing victim, but she’d been stabbed in the kitchen, with a knife from her own drawer. She’d fought, she’d died bloody, and the killer had left her there, her skirt on sideways, her body contorted.

You need to see them, I told the killer silently, picturing his silhouette in my mind. You need them to see you. They need to be beautiful.

This third victim had been killed after the first two. The UNSUB’s MO had changed: different weapon, different location. But deep down, the killer hadn’t changed. He was still the same person with the same sick underlying needs.

Every time you kill, you need more. You need to be better. She needs to be better. Killing women on the street wasn’t enough anymore. You didn’t want a quickie in a back alley. You wanted a relationship. A woman. A home.

I zeroed in on the two women who’d been killed in their bedrooms. Both had been found lying on their beds. One had been shot. The other had been strangled.

You catch her at night. In her house. In her bedroom. She doesn’t look through you now, does she? She’s not too good for you now.

I tried to imagine the UNSUB shooting a woman, but the math on that one just did not compute.

You want her to see you. You want to touch her. You want to feel the life going out of her, little by little.

“This was the last one,” I said, pointing to the woman who’d been strangled in her own bed. “Different MO. Same signature.”