"I'm not holding back, Detective. Data is spa.r.s.e." She arched a brow at him, and he said, "But I'll happily go over it again. I was FBI then and was made point liaison with your mother when she reached out to the Bureau to say she knew of a threat to security within our borders. She told me she had been developing an informant within a terror group, and we funded her two hundred grand to bribe her insider in exchange for proof and details of the plot. We gave your mother the money for the transaction the day she was murdered."

Heat had already known that much. But she now wanted to ask some new questions. "Did you know who her informant was?" When the agent shook no, she said, "I believe it was a man name Ari Weiss. Deceased now, so no help. But he was college friends with a Brit living here named Carey Maggs." She searched his face for recognition of either name and got none. "Would it be a pain to ask you to run a check on Maggs for me?"

"You think he might be involved?"

She shrugged. "Doesn't seem to be. But I like both belt and suspenders, you know?" Callan twisted open his gold Cross ballpoint and wrote the name down. When he'd finished, she asked, "What about the two hundred thou? Did it ever surface? I know you guys had to mark the bills."

He wagged his head again. "End of our intel, end of story." Then he added, "Well, it was the end of the trail until you exposed Tyler Wynn. Which is why I am renewing my pitch to you. The memos out of DC call it cooperative interface. Join me, Nikki. I have resources. We'd make a great team." He started to reach a hand across the table, but she casually slid hers onto her lap.

"Thanks, but I do better independently."

He waved his hands back and forth between the two of them. "Then what do you call this?"

"Cooperative interface. And your NYPD appreciates it."

Out on Madison Avenue, she declined his offer of a ride, even though it would have afforded her a measure of security with Salena Kaye on the hunt for her. The agent said fine, but reminded her that if she ever wanted a sparring partner, he'd be game. From her taxi, Nikki glimpsed him getting into a black Suburban with US government plates, and figured Bart Callan could give her quite a workout.

Detective Heat closed her eyes and ran her math. The equation began with Callan's intel that her mom had been cultivating an informant. He couldn't name the insider, but with the new connection Nikki had drawn from her mother to Ari Weiss as a member of Tyler Wynn's circle, she didn't have to be at a blackboard in Good Will Hunting to surmise that Cynthia Heat had not been spying on Dr. Weiss-she was cultivating him as a snitch.

She pulled up his obituary on her iPhone. The date of his pa.s.sing from a tick-borne illness was January 2, 2000. Only six weeks after her mother's death.

As soon as she locked her apartment door behind her, Heat speed-dialed the home of a judge she'd met at one of Rook's weekly poker games. After Judge Simpson razzed her about giving him a chance to win his money back, Nikki asked him a favor: to write a court order for the exhumation of Ari Weiss.

Royksopp startled her from the computer screen. After Rook's call that morning from Nice, his ringtone, a song from a caveman commercial, seemed newly appropriate to Nikki.

"It's late there. I was afraid you'd be in bed," said Rook.

"I'm going over squad reports on my serial killer."

"I'm in London. Heathrow, actually. Workin' my way back to you, babe." The joker, trying to laugh it off, she thought during the long silence she fed him. "Should be there by sunup, your time. I'm flying Virgin."

"I doubt that." Another dose of awkward pause.

"Nikki, I guess I can see why you got all bent about Yardley, but you're reading way too much into this."

"Am I?"

"Yes." They listened to each other breathe. Then he said, "They're calling my flight."

"How long is it?"

"Let's see, uh... a little over seven hours."

"Good," she said. "Use it to work on your empathy."

Detective Heat brought her crew in for another early roll call the next morning. This time, they were joined by Detectives Malcolm and Reynolds, on loan from the major case squad. They were quick studies, so Nikki only needed to use the first ten minutes to recap the two murders and get them up to speed. As she wrapped it up, Sharon Hinesburg slid into the back of the bull pen, the only detective to be tardy.

Traces on the physical evidence from each homicide scene had brought no results after a day and a half of calling and canva.s.sing. The red and yellow string was so common and widely available that screening recent purchases could take weeks, plus it could have been bought months or years ago. Same, too, with the skate wheel.

Malcolm raised a hand. "Let me tell you something." He slouched back in his usual pose and planted one of his work boots on the back of a chair. "Coming in cold?... Whenever I come across props like this in a case, it's one of two things. Either there's some sort of personal c.r.a.p the guy's working out..."

"You mean like fetishes?" asked Heat.

"Yeah, or some f.u.c.ked-up, brain-fried, thumb-sucking obsession like his mommy wouldn't let him have pets or ride a skateboard."

"... While carrying scissors," added his partner, Reynolds.

"Or second, he's just seeding chaff to mess with our heads." Malcolm brought his cup up to sip. "Who knows?"

"Only the killer," said Heat. "Let's keep on tracing those items, especially the string, which is common to both, but keep digging on the victims. People in their lives, how they spent their last day, and especially-are they somehow connected to each other beyond their job types?"

Detective Raley reported that only one neighborhood camera was pointed at the Maxine Berkowitz crime scene. "It's outside a neighborhood Islamic center on Riverside Drive," he said. "And it's out of order."

Heat logged that in marker on the Berkowitz whiteboard, then tapped the identical notation for the pizza joint cam in the other murder. "Coincidence?" she said. "I would say strange enough to be considered..."

"Wait for it," called Feller.

"... an odd sock," said Nikki, and the room erupted in a chorus of "Yessss!" at the first invocation of Heat's pet investigative phrase on this case. But the rowdiness was quelled when one of the administrative aides brought in the morning papers and held one of the tabloids up to the room. The bold headline screamed: DEAD TIE! Underneath, against a white background, blared a giant photo of two coils of string: one red, one yellow.

Heat dismissed the meeting, and the rest of the squad did exactly what she did: They dove into the New York Ledger. "Exclusive," read the subhead, and the byline was Tam Svejda, Senior Metro Reporter for the Ledger-whom Heat knew, among other things, to be a lazy journalist p.r.o.ne to easy handouts from "insiders." Detective Hinesburg had whispered confidential material to her before, acting as Captain Irons's mouthpiece-an apt term, considering her s.e.xual relationship with the skipper. To Nikki the article felt warmed over, derivative of old reports already made public. But then there was the leak of the big hold-back: that the two homicides were literally bound together by string, which pointed to a serial killer operating in Manhattan.

"Now, calm down, Detective," said Wally Irons. Heat appeared in his office before he could set down his briefcase. "We were going to release that today anyway."

"But we didn't. Someone leaked it. And whoever it was put our MO hold-back on page one," she said, brandishing the picture of the string.

"First things first," he said, seeming to enjoy this. "Tam Svejda called me for comment, and you can see for yourself, I downplayed the serial killer angle. Here it is." He ran a finger down the column and quoted, "Precinct Commander Captain Wallace Irons cautioned against leaping to conclusions. 'We cannot rule out the possibility that these killings could be the work of separate individuals.' "

"n.o.body's going to buy that," said Heat.

"Ah, but it's on the record. I did my part."

Nikki slapped her thigh with the tabloid, wondering how she had gotten so lucky to work under the Iron Man. Detective Ochoa stuck his head in the door.

"Excuse me, Detective? Got a call on your line from a guy who says he's the serial killer."

"See?" Nikki shook the newspaper at Irons. "The bogus calls are starting already."

But then Ochoa said, "Detective Heat? He asked if you liked to roller blade."

Heat tossed the tabloid on a guest chair and rushed out to her desk.

FIVE.

"This is Detective Heat."

"Got your attention, did I?" The voice sounded male, but distorted, the way 20/20 electronically disguises voices of mob witnesses and whistle-blowers.

"It's a start," said Nikki. She sat at her desk, and when she swiveled in her chair, she saw that the entire squad had gathered around her. "So. Tell me what you're calling about." There was a loud click and the line went dead. She stared at the phone and had started to tell the others he'd hung up when her line rang. She jumped on it. "Heat."

The distortion made him sound even more chilling. "Do not f.u.c.k with me. Pull that casual chatty bulls.h.i.t again, I'm gone. Got it?"

"Got it."

"I'm going to talk, and you're going to listen." Nikki looked over at Raley where he coordinated the call trace at his desk. "What's this s.h.i.t in the paper about it could be two people? Do I have to prove it's not?"

"No," she said immediately.

"We'll see. I get to decide that, cover girl." All of her training had taught Heat to remain dispa.s.sionate in these kinds of calls. But her heart b.u.mped at the reference to her magazine cover. She tried to bat away the personalization. He had other ideas. "Think you're so smart, Detective Heat? How smart do you feel running around like a rat in a maze? You smell a clue but you can't see it. You need something to unlock that door."

Nikki wanted to keep him talking, not just for the trace but to get a handle on him. "You don't have to make this a contest."

"Sorry." Then he laughed, a digitally altered Darth Vader. "Tell you what, cover girl. Maybe I'll give you a hand on the next one." And then he clicked off again. Heat stood to look over the other detectives at Raley, who shook his head and hung up his phone.

Nikki went into the restroom and splashed water on her face again and again. It just seemed like something to do when all she wanted was to be alone. Drying off, she felt the paper towel tremble in her hands as she took in the magnitude of what had just happened. A challenge had been laid down. An already baffling case had suddenly taken on a new dimension for Heat, who now found herself matching wits against a serial killer, with innocent lives at stake over how good she really was. "Cover girl," she muttered into her hands. Nikki peeled the wet towel off her face, chucked it, and left the room without so much as a glance in the mirror.

When Heat came back into the bull pen, she found another unsettling surprise waiting. "Je suis retourne!" Jameson Rook slid off her blotter and stood beside his roll-along bag. Grinning through traveler's stubble, he held his arms open wide as she approached. She wouldn't ice him in public, but the hug Nikki gave him wouldn't exactly have lighted up the Kiss Cam at the Garden. "Brr," he said in a low tone. Then added, "See, I've been working on my empathy."

"Not the best time, Rook."

"Let me guess." He held up his copy of the Ledger. "I saw this in the airport when I got off the plane."

Raley walked by, holding out a transcript of the phone call. She made a no-look s.n.a.t.c.h as he moved on, distributing it to the squad as they a.s.sembled around the Murder Boards. "The serial killer reads the Ledger, too, and he just called."

"You spoke to him?"

"I did."

"Then I got back just in time." He breezed past her and took an empty seat with the detectives. Determined to ignore this new distraction, Nikki took her place up front.

"An a.s.signment," said Heat as she surveyed the room. "I need someone out at Reception to monitor incoming calls so if our serial killer tries me again, he gets right through." Her gaze fell on Detective Hinesburg. "Sharon, you're elected."

Hinesburg made the face of snippy annoyance. "Fine. Your party."

"You're right," said Nikki, who waited for Hinesburg to saunter off to the precinct lobby, figuring that if the detective was out of earshot, she couldn't learn anything to leak to the paper. Heat addressed the rest of the group. "Before we begin, has anyone not read this?" She held up her copy of the tabloid.

After a moment of silence Ochoa said, "Want me to ask Detective Hinesburg?"

When the squad's knowing laughter settled, Heat said, "Yeah, I have a feeling Sharon's caught up." She waited out a few more chuckles then brought them to business. "Most of you heard my side of the two calls we just got. And you've all got the transcript. Detective Raley also has dubbed an audio copy off our digital call server. Rales?"

He opened the WAV file on his laptop speakers. At first, Rook and the detectives started to read along. But as the chilling call continued, enticingly sinister because of the digitally futzed voice, they all abandoned their hard copies and leaned forward, staring instead at the computer, as if it were the man himself instead of the playback device for a killer's audio bit stream. When it finished, Detective Raley clicked it off.

Complete silence followed.

Heat broke it by asking, "OK, what did we learn?" She bisected the Maxine Berkowitz Murder Board with a vertical line and began a brainstorm list in the open white s.p.a.ce.

"It's him," said Detective Feller. "He worked in the hold-backs that didn't get leaked: the skate reference and the rat in the maze thing? It's him."

"For now, let's say so," Heat agreed, and saw bobble-heads.

"Tech-savvy," said Detective Reynolds. "Not everyone out there knows how to alter his voiceprint like that."

Rook couldn't resist. "There's an app for that?"

"Raley," said Heat. "As my King of All Surveillance Media, find out if there is." Rales nodded and made a note. "What else?"

"Dude's controlling," called out Ochoa.

Heat said, "No kidding," and wrote the trait on the board. "The way he hung up on the first call to let me know who's boss."

"And the second call," added Rook. "It was all about making his points his way, in his own time, like a billiard champ running the table."

Detective Rhymer said, "I'd put smart up there, too." As Nikki posted that, he continued, "He knew exactly how long to stay on the call to beat the trace, and he also knew how to push your b.u.t.tons, talking about case frustration..."

"... Calling you a cover girl," said Reynolds. Nikki's eyes went to Rook's and then away.

"I think this guy's beyond smart and controlling," said Malcolm. "I say he's p.i.s.sed. Check it out." He skim-read from the transcript, " 'Do not f.u.c.k with me.'... 'I'm going to talk, and you're going to listen.'... 'Think you're so smart, Detective Heat?' "

"That's not just p.i.s.sed," said Raley.

"That's compet.i.tive," finished his partner. "Talking about making it a contest, and maybe 'helping you' with the next one."

"That's the biggest clue of all," said Heat. "And the worst." She didn't have to voice it. The caller already had-that there would be a next one.

Later that morning, Roach came to Nikki's desk. "Rook was right," said Detective Ochoa.

"There is an app for that." Raley picked up. Across the room at his squatter's desk, Rook overheard and came to join them as the media king briefed Heat. "There's not only an actual app, but we found a slew of consumer software out there for altering voices. All you need is a laptop to change how you sound."

His partner continued, "You can do the Darth Vader like our man, or girls can sound like old ladies, or men can pretend to be women..."

Rook jumped in. "That's why I always say..."

" 'Check the Adam's apples,' " said Roach in a singsong chorus.

Heat stayed on task. "So this is all widely available?"

"Maybe not as much as skate wheels and string," said Raley, "but close. Plus a hobbyist could probably go to his neighborhood Radio Shack and find all he needed to build his own electronic voice box."

"Then we start calling Radio Shacks." As Nikki said it, she knew-and they knew-it could be tail chasing. The kind of thing she'd put Sharon Hinesburg on. "We have to take every shot."

They split off to work it, and she called after them, "And ask Detective Rhymer to reach out to the app vendors." To Heat's irritation, Rook stayed put. "A little busy," she said, picking up a report.