For the whole time he had been in custody, his demeanor had not changed. Though she found it hard to understand why, he was still casual and cheery, as if he believed that this was all a comic misunderstanding and easily cleared up. The only thing that had changed over the hours they held him was the beginning of a shake in his right hand, a telltale sign of an alcoholic in need of their next drink.

Maybe that was a weakness that she could use, at least.

“I am going back in,” Zoe announced. She had grabbed up the files holding the crime scene photographs—specifically, the equations.

“Do you want me with you?” Shelley asked. She, too, had been watching for any kind of sign, while they chased surveillance footage from areas around his apartment over the phone. So far, nothing had shown him leaving his apartment. It didn’t mean that he hadn’t slipped by in an area not covered, but it did mean they had nothing to threaten him with.

“No.” Zoe made for the door, buoyed along by a new determination. “You watch him. Closely.”

“Call him professor,” Shelley called after her. “You’ll stroke his ego. False sense of security.”

They couldn’t keep him at the field office for long. A long time for him, surely, but in terms of their investigation, not long enough. If he didn’t crack soon, they would have to let him leave. So, she would have to make him crack.

Zoe entered the interrogation room and resumed her seat opposite Wardenford, who greeted her with a cheery smile.

“Time to let me out yet, Agent?”

“Not yet.” Zoe paused, opening the folder at such an angle that only she could see the contents. “How are you with math, Professor?”

Wardenford seemed to swell with ego as she gave him his former title. Shelley had been right. “It’s one of my specialties,” he said. “Of course, math goes hand in hand with physics. It’s been my life’s work.”

Zoe nodded. “I understand,” she said. “Then perhaps you can help us out with something? We have some equations that we are trying to figure out.”

She first reached for the printouts she had created: the equations alone, copied out on the computer, rather than the crime scene photographs. No blood, no sign that they had anything to do with the killings. She laid them down one by one in front of him, watching his face as he leaned forward to study them.

There was no flicker of recognition in his eyes, at least not that Zoe could see. She glanced up toward the black glass wall, as if she could see through and divine what Shelley was thinking. Of course, there was nothing to see there.

Back to Wardenford; he was lifting the printouts in his hands now, comparing them side by side, rubbing his mouth and resting with his fingers over it as he leaned on his elbow. He spent longer looking at the first than the second. He frowned deeply, then deeper, the furrows on his brows lengthening and sinking.

Minutes stretched on. Zoe kept count of them: four, six, ten. He was still staring at the equations, shifting in his seat sometimes, even mouthing things to himself as he worked through them. Zoe let the silence continue, not wanting to interrupt. What he said and did now was important.

“They’re unsolvable,” he declared at last, throwing the two pieces of paper down onto the desk. “This is some kind of trick, isn’t it?”

“Trick?” Zoe raised an eyebrow.

“You think if you can frustrate me with an equation I can’t solve, I will be vulnerable to questioning and end up admitting everything. Well, I can’t admit anything. I didn’t do it.”

“This is not a trick, Professor,” Zoe said, opening her folder on the desk and spinning it toward him so that he could see it. Inside, the images were piled haphazardly: the equations scrawled out on torsos, blood, close-ups of the injuries to the heads. “We really do need to figure out those equations.”

At last, there was a reaction on Wardenford’s face. Not the kind of reaction that Zoe had been hoping for—a microscopic twitch, a flinch, a tiny tell that would give him away. Patterns were easy to spot in human behavior. There should have been something that told her he knew what he was looking at, and he was lying.

But there was nothing there. Just revulsion. Wardenford paled, gasped, covered his mouth. Finally, he squeezed his eyes shut and moved his head away so that he no longer had to look at them. “That’s horrible,” he finally managed. “Cole and—and Ralph. God. Who could do something so violent?”

“The same person who wrote out those equations.” Zoe tapped the paper in front of him, drawing his attention back. “So tell me, Professor. Help us. What do they mean?”

Wardenford stole a glance at the crime scene shots and shuddered before looking down at the paper. Zoe had seen that before. People would look again and again at things they found disgusting or distressing. They couldn’t help themselves.

Of course, people also looked again and again at things they were proud of.

“They don’t mean anything,” Wardenford said. His face was ashen now, and the cheeriness was gone entirely. In that, at least, Zoe had achieved her goal. “They ought to, but something’s wrong. It’s like all of the elements are there, but they’ve been placed incorrectly. Imbalanced. Too much on one side, not enough on the other. You won’t be able to solve them or find out what they mean. They’re wrong.”

Zoe sighed, slumping back into her chair. It was the same as the others had said. Dr. Applewhite and her colleagues had been right, and she hadn’t wanted to admit that. But it was getting harder and harder to deny that these equations weren’t solvable.

At least, not yet.

“Imbalanced,” she repeated, her brain starting to work.

“Yes,” Wardenford confirmed. “See, here? This one should really have something there, but there’s nothing. It doesn’t make sense this way.”

Imbalanced… what if that was the whole point? What if these were not individual equations to be taken separately, but part of a larger puzzle?

Zoe thought back to their last major case, to the Golden Ratio killer. His plans had seemed different at the beginning. It was only as he filled in more points on the map, took more victims, that the spiral shape became clear.

That was a terrible thought—that she might need more information. Need another death. But it did make a lot more sense than what they had already—which was nothing.

Zoe spun the two printouts toward herself and grabbed a pen from her pocket. She started to balance the equations out—adding them together. It was easy to see the spaces, now she understood to look for that kind of pattern. And it was easy to see the things on the second equation that stood out, begged to be put somewhere else.

She worked in a frenzy, forgetting that Wardenford was even in the room. This was more important than the interrogation. If he was right, this could change everything. Maybe they could work something out from this, some kind of formula, or a prediction of what the next equation would be. Any little clue along the way could help them figure out who the killer was.

That was, of course, assuming that it wasn’t Wardenford and he wasn’t stringing her along like a puppet, watching her dance.

Zoe paused then, looking up to see that Wardenford was watching her. Closely. She stopped writing. Perhaps that thought was right. Perhaps she was playing right into his hands, taking the bait.

“You don’t see things like others do, do you?” he asked, unexpectedly.

“What?”

“I’ve met people like you before. You’ve got a way with numbers and patterns, am I right? You’re a synesthete.”

Zoe instinctively looked toward the darkened glass, hoping the tech had left the room. If Shelley was the only one hearing this, it wouldn’t be so bad. But this was on record. Taped. Anyone could see it. She fought a rising sense of panic, her hand flying up to just below her collarbone, her neck. She felt that same stifling feeling that came when she sat in the passenger seat and the seatbelt seemed to choke her, but there was nothing there to pull away.

“I knew it. You remind me exactly of someone I mentored years ago.”

Zoe was torn between anxiety over her secret being outed, and the shock that he could tell just by looking at her. “What are you talking about?” she asked, hoping it would sound like a deflection but also prompt him to explain how he had known.

“I know brilliance when I see it. You have an instinctive way of working with the numbers, and it’s not just that. You’re constantly assessing things, sizing them up. I can recognize it because I’ve seen it before.”

“With your student,” Zoe replied, which was not an admission, but still encouraged him to go on all the same. She was walking a dangerous line. If anyone saw this, she would have to flat-out deny it—or come clean. At least not having the admission on tape was a slim comfort.

“Yes. She was gifted—just gifted. I noticed her skills in class, and invited her for some extra sessions to see if we could coax out that genius. Lo and behold, she had capabilities I had never dreamed of. To look at a math equation and know the answer, just like that.”

“What happened to her?” Zoe was desperate to know. After the news Dr. Applewhite had told her, of the student committing suicide, it was of the utmost interest to her. Had she been successful in life? Started a family, maybe?

“Ah, well, I don’t really know.” Wardenford coughed quietly, wearing an embarrassed expression. “I ended up quitting, you see. Coming over here to work instead. That was after my divorce; I had to get away. All my problems started there.”

“That is when you began drinking.”

“Right.” Wardenford sighed heavily. “That’s the part of the job I miss the most, you know? Nurturing young minds, helping them come to their full potential. Like you—putting the skills and talents they have to good use. Helping them to figure out what to do with the rest of their lives. I suppose all that is gone, now. No college anywhere near here is going to touch me, and I doubt I’ll have a good reference if I apply elsewhere.”

Maudlin self-pity. Zoe was just about to tell him to shut up and stop feeling sorry for himself, and go work on getting the things he wanted instead of drinking himself to death. Perhaps happily for her career, that was the moment that Shelley threw open the door and interrupted instead.

“Agent Rose,” Zoe remarked, surprised that she would break protocol by entering the interview room. Perhaps one of their superiors had arrived, and Shelley had come to warn her…?

“Agent Prime, a word, please,” Shelley said, moving back into the corridor to let Zoe out.

Once the door was firmly closed behind her and Wardenford was out of earshot, Shelley brandished her phone, indicating the source of the news that was spilling out of her. “They’ve found another body.”

Shelley’s words rolled over Zoe like a wave. There was another death. It had probably happened while Wardenford was in custody, which would mean that he was innocent.

But maybe it held another equation—another clue.

Zoe didn’t know whether to be pleased or dismayed. Their phantom math killer had struck again.

But that meant there was a whole wealth of more clues waiting, any of which might help them catch him and stop him in his tracks.

CHAPTER FIFTEEN

Zoe hit the brakes, almost sending the car into a skid. She had been driving so fast down the wide, leafy suburban streets that she had almost missed the police car parked up ahead and gone right into the back of it.

They had landed outside a huge Georgian colonial, not at all out of place in this expensive neighborhood. The one thing that did set it apart were the white-suited forensics experts and uniformed police bustling outside or rushing in and out of the door in a near-perpetual routine.

Shelley was already out of her seatbelt and the door by the time Zoe had turned the engine off, and she wasn’t far behind her. They both ran across the neatly kept lawn to the entrance, flashing their badges quickly at the policeman who tried to stop them approaching from the sidewalk.

The commanding officer at the scene met them at the door, knowing from a glance that they were the FBI agents he had been told to wait for.

“Agents, you’re going to want to come and see this. It’s a brutal one. Looks like another one of our math killer’s hits.”

They followed him hurriedly up a wide staircase to a master bedroom, dodging other personnel who were coming and going with fingerprint kits and DSLRs and spare evidence bags. Zoe had already counted thirteen pairs of boots on the ground. This was clearly a big deal to the locals—and of course it would be. When wealthy neighborhoods were home to violent and brutal murders, it was normally in the interest of the sheriff or chief of police to do something about it, and fast.

“Cleaner called us in when she reported for work and found the body. Thankfully she was in the habit of speaking to her employer first rather than getting right to it, so she didn’t wipe any evidence away. The vic is a neurologist from the local hospital, Dr. Edwin North. Pretty well-known around these parts. He and his wife used to take part in all the community events, you know? Real pillars. His late wife, that is. Cancer last year.”

This running commentary was given as they ascended the stairs, and the officer paused them outside the room itself. “I’ve got to tell you ladies, this is a real bad case. Maybe you shouldn’t go in there. We’ll have the crime scene photos along to you, but you might be better off not seeing it in person.”

“We’re not ladies,” Shelley said, brushing by him. “We’re federal agents, and I assure you we can handle it.”

Zoe held back a laugh at the man’s expression, and followed her. What she saw was not at all pretty. Shelley must have been fighting hard not to show any reaction, given how emotional she normally was at crime scenes.

The doctor’s head was crushed, visibly so. There was an odd shape to his head, newly formed after his skull gave way under the pressure. Oblong, distorted. His eyes had bulged out under the force, his eye sockets broken at their upper edge. Brain matter and blood, along with fragments of skull, decorated the headboard.

He was lying in bed, alone, still partially covered by a blanket. He was half-dressed, giving the impression that he had stolen into the bedroom for a quick nap and nothing more. It was a nap that he was never going to wake up from again.

But most exciting of all was the link that Zoe had been waiting for. His plain shirt had been ripped open, traces of blood still clinging to it where it had been thrown aside. Across his bare torso, another equation was written in thick black numbers and letters.

The blood was still wet. He had been killed in the last hour or less. Even as they watched, a small piece of brain matter that had attached itself to the wall slowly peeled away and dropped down. This crime scene was still settling into place.

This had happened while they were at James Wardenford’s home, arresting him, or at least in the minutes before or after. No way he could have got home, washed himself off, and played the part of the drunkard in time. Even the first part would have been too much of a stretch, given the distance between the homes. Wardenford was in the clear.

Shelley was taking it all in, breathing through her mouth rather than her nose, and Zoe took that as her cue. She was long used to gruesome scenes like these, and it was all just meat to her. Better that she take the lead while Shelley found her feet.

“What was his schedule for today?” Zoe asked.

The policeman flipped back a page in his notebook. “He finished his last shift early this morning, and then was due back in this evening at nine to handle a late shift. Looks like he was getting some shut-eye beforehand.”

Shelley had recovered enough to draw closer to the body. “Any initial forensics reports?”

The officer followed her, leaning in to point at various parts of the skull with an outstretched pen. “They tell me the doctor was stunned first with a single blow, here. We can only just see the edge of the impact mark under all the rest of it, but it was likely solidly across the front of his head. Enough that even if he woke up, he’d have been out of it. Difficult for him to fight back at all.”

Shelley nodded, while Zoe ranged around the room, careful where she stepped. She was making calculations. She knew it took around a thousand pounds of force to cause the average skull fracture. Their killer certainly was not heavy or strong enough to provide that force himself—so he must have used something heavy, and thrown it down on top of the victim’s head.

“Have you recovered the weapon?” she asked.

Their guide, as well as the three forensics officials still working in the room, all shook their heads.

“Heavy, but thin,” Shelley suggested, studying the impact marks on the man’s face.

Zoe nodded approvingly. “No wider than my hand. Dropped three or four times, with decreasing force each time. Our killer was running out of strength.”

“Then, did he bring it with him? Or take it from the house?”

Zoe thought that over. “Interesting question. Either he planned in advance very carefully, or he took an opportunity when he found one. What do we think?”

“He seems like both. It’s a paradox, this case. Planned and premeditated—he waited for the professor. Took the student somewhere that wasn’t covered by surveillance. But the killings themselves are rage-driven, spontaneous. Using the environment.”

“How did he get in?” Zoe directed her question at the officer.

“Back door had been sabotaged. It’s almost as old as the house, beautiful wood paneling. Someone carefully and slowly sawed through it, put their hand through one of the panels, and turned the key from the outside to let themselves in. The doctor had ambient noise playing in here over his smart speakers. He wouldn’t have heard a thing, I don’t think.”

Zoe was done with the scene; she knew everything she needed to, from there at least. Nothing contradicted her earlier thoughts that the killer would be five foot nine, of average build, but perhaps a little muscular. Now she could let herself indulge in the one thing she was really interested in.

She took out her phone and started taking photographs of the equation, angling herself in to get the best shots. Shelley and the others in the room continued low conversations, but Zoe tuned them out, only keeping herself vaguely aware of what they were saying in case something important came up.

The shots taken, Zoe drifted down out of the room and down the corridor, lightly nudging the next door open with her elbow so as to avoid touching the handle. There was light afternoon sun streaming through the large windows, illuminating a miniature gym room with a treadmill set up to face the view.

Zoe moved past it, examining the other items. A large blue balance ball, several straps and lengths of stretchy material used for strength training. A rowing machine, low on the ground, with an empty water bottle still fitted into the appropriate slot.

Weights, stacked up against the far wall. Zoe counted their number and value, noted the thicker layer of dust on the bottom weights—the heaviest—compared to the top. By the pyramid-like stand was a bar, the kind you thought of when you imagined old-fashioned weightlifters. There were several flat, circular weights stacked beside it, evidently used to increase or decrease the weight on the bar as you wished.

Zoe crouched, her attention suddenly caught by something. And yes—there it was. On the edge of one of the larger weights, concentrated much more in one area with almost nothing further along the circle. Blood and fragments of brain and skull.

“In here!” she shouted, sure now that she was looking at the murder weapon.

Shelley arrived fast, the forensics people hot on her heels. Zoe moved out of the way to allow them in, pointing out the incriminating evidence. She looked over the rest of the room more closely, seeking another sign of their killer’s presence. A footprint in undisturbed dust, a smudge from a finger, anything that would help.

“What are these here?”

Shelley’s voice snapped Zoe out of her concentration. The forensics team rushed over to where Shelley was pointing, on the floor just by the dropped weight.

“Strands of hair,” one of them muttered, taking out an evidence bag. “Very short. Well spotted.”

“It could be the victim’s,” another of them pointed out, his voice muffled by the mask he wore over his mouth.

“One dark and one gray,” Shelley said. “The victim looks to have been blond. From here, at least.”

The two hairs were lifted with fine tweezers, dropped into the waiting evidence bag, and marked. “We’ll have them analyzed. With any luck, there will be enough of the hair follicle on there to get us a match.”

“We might have our killer,” Shelley said, with such obvious glee that it sent a thrill up Zoe’s spine. She was right. That kind of break could crack the whole case wide open, give them a name. Once they had that, they could get him in for questioning, get him to tell them everything. Hairs weren’t always worth what they used to be in a courtroom, but a confession was.

And this was exactly the kind of evidence that Zoe knew Shelley could put to good use in extracting a confession.

All the tools they needed to close the case may already have been sitting in that evidence bag.