She opened the door to Peter’s room and set her mug on the dresser so that she could make his bed. What’s the point, Peter would say when she dogged him to do it himself. I just have to mess it up again in a few hours.

For the most part, she didn’t enter Peter’s room unless he was in it. Maybe that was why, at first, she felt there was something wrong about the space, as if an integral part were missing. At first she assumed that it was Peter’s absence that made the room seem a little empty, then she realized that the computer-a steady hum, an ever-ready green screen-had been turned off.

She tugged the sheets up and tucked in the edges; she drew the quilt over them and fluffed the pillows. At the threshold of Peter’s bedroom she paused and smiled: the room looked perfect.

Zoe Patterson was wondering what it was like to kiss a guy who had braces. Not that it was a remote possibility for her anytime in the near future, but she figured it was something she ought to consider before the moment actually caught her off guard. In fact, she wondered what it would be like to kiss a guy, period-even one who wasn’t orthodontically challenged, like her. And honestly, was there any place better than a stupid math class to let your mind wander?

Mr. McCabe, who thought he was the Chris Rock of algebra, was doing his daily stand-up routine. “So, two kids are in the lunch line, when the first kid turns to his friend and says, ‘I have no money! What should I do?’ And his buddy says, ‘2x + 5!’”

Zoe looked up at the clock. She counted along with the second hand until it was 9:50 on the dot and then popped out of her seat to hand Mr. McCabe a pass. “Ah, orthodontia,” he read out loud. “Well, make sure he doesn’t wire your mouth shut, Ms. Patterson. So, the buddy says, ‘2x + 5.’ A binomial. Get it? Buy-no-meal?!”

Zoe hefted her backpack onto her shoulders and walked out of the classroom. She had to meet her mom in front of the school at ten o’clock-parking was killer, so it would be a drive-by pickup. Mid-class, the halls were hollow and resonant; it felt like trudging through the belly of a whale. Zoe detoured into the main office to sign out on the secretary’s clipboard, and then nearly mowed down a kid in her hurry to get outside.

It was warm enough to unzip her jacket and think of summer and soccer camp and what it would be like when her palate expander was finally removed. If you kissed a guy who didn’t have braces, and you pressed too hard, could you cut his gums? Something told Zoe that if you made a guy bleed, you probably wouldn’t be hooking up with him again. What if he had braces, too, like that blond kid from Chicago who’d just transferred and sat in front of her in English (not that she liked him or anything, although he had turned around to hand her back her homework paper and held on to it just a smidgen too long…)? Would they get stuck together like jammed gears and have to be taken to the emergency room at the hospital, and how totally humiliating would that be?

Zoe ran her tongue along the ragged metal fence posts in her mouth. Maybe she could temporarily join a convent.

She sighed and peered down the block to see whether she could make out her mom’s green Explorer from the conga line of passing cars. And just about then, something exploded.

Patrick sat at a red light in his unmarked police car, waiting to turn onto the highway. Beside him, on the passenger seat, was a paper bag with a vial of cocaine inside it. The dealer they’d busted at the high school had admitted it was cocaine, and yet Patrick had to waste half his day taking it to the state lab so that someone in a white coat could tell him what he already knew. He fiddled with the volume button of the dispatch radio just in time to hear the fire department being sent to the high school for an explosion. Probably the boiler; the school was old enough for its internal structure to be falling apart. He tried to remember where the boiler was located in Sterling High, and wondered if they’d be lucky enough to come out of that kind of situation without anyone being hurt.

Shots fired…

The light turned green, but Patrick didn’t move. The discharge of a gun in Sterling was rare enough to have him narrow his attention to the voice on the dispatch radio, waiting for an explanation.

At the high school…Sterling High…

The dispatcher’s voice was getting faster, more intense. Patrick wheeled the car in a U-turn and started toward the school with his lights flashing. Other voices began to transmit in static bursts: officers stating their positions in town; the on-duty supervisor trying to coordinate manpower and calling for mutual aid from Hanover and Lebanon. Their voices knotted and tangled, blocking one another so that everything and nothing was being said at once.

Signal 1000, the dispatcher said. Signal 1000.

In Patrick’s entire career as a detective, he’d only heard that call twice. Once was in Maine, when a deadbeat dad had taken an officer hostage. Once was in Sterling, during a potential bank robbery that turned out to be a false alarm. Signal 1000 meant that everyone, immediately, was to get off the radio and leave it free for dispatch. It meant that what they were dealing with was not routine police business.

It meant life or death.

Chaos was a constellation of students, running out of the school and trampling the injured. A boy holding a handmade sign in an upstairs window that read help us. Two girls hugging each other and sobbing. Chaos was blood melting pink on the snow; it was the drip of parents that turned into a stream and then a raging river, screaming out the names of their missing children. Chaos was a TV camera in your face, not enough ambulances, not enough officers, and no plan for how to react when the world as you knew it went to pieces.

Patrick pulled halfway onto the sidewalk and grabbed his bulletproof vest from the back of the car. Already, adrenaline pulsed through him, making the edges of his vision swim and his senses more acute. He found Chief O’Rourke standing with a megaphone in the middle of the melee. “We don’t know what we’re dealing with yet,” the chief said. “SOU’s on its way.”

Patrick didn’t give a damn about the Special Operations Unit. By the time the SWAT team got here, a hundred more shots might be fired; a kid might be killed. He drew his gun. “I’m going in.”

“The hell you are. That’s not protocol.”

“There is no fucking protocol for this,” Patrick snapped. “You can fire me later.”

As he raced up the steps to the school, he was vaguely aware of two other patrol officers bucking the chief’s commands and joining him in the fray. Patrick directed them each down a different hallway, and then he himself pushed through the double doors, past students who were shoving each other in an effort to get outside. Fire alarms blared so loudly that Patrick had to strain to hear the gunshots. He grabbed the coat of a boy streaking past him. “Who is it?” he yelled. “Who’s shooting?”

The kid shook his head, speechless, and wrenched away. Patrick watched him run crazily down the hallway, open the door, burst into a rectangle of sunlight.

Students funneled around him, as if he were a stone in a river. Smoke billowed and burned his eyes. Patrick heard another staccato of gunshots, and had to restrain himself from running toward them blindly. “How many of them?” he cried as a girl ran by.

“I…I don’t know…”

The boy beside her turned around and looked at Patrick, torn between offering knowledge and getting the hell out of there. “It’s a kid…he’s shooting everyone…”

That was enough. Patrick pushed against the tide, a salmon swimming upstream. Homework papers were scattered on the floor; shell casings rolled beneath the heels of his shoes. Ceiling tiles had been shot off, and a fine gray dust coated the broken bodies that lay twisted on the floor. Patrick ignored all of this, going against most of his training-running past doors that might hide a perp, disregarding rooms that should have been searched-instead driving forward with his weapon drawn and his heart beating through every inch of his skin. Later, he would remember other sights that he didn’t have time to register right away: the heating duct covers that had been pried loose so that students could hide in the crawl space; the shoes left behind by kids who literally ran out of them; the eerie prescience of crime-scene outlines on the floor outside the biology classrooms, where students had been tracing their own bodies on butcher paper for an assignment.