One April morning in 2014, a sixteen-year-old sophomore at Franklin Regional Senior High, in Murrysville, Pennsylvania, stole two butcher knives from his parents’ kitchen, hid them in his backpack, and took them to school. He was wearing all black and, according to witnesses, had a “blank expression.” Just before first period, in the hall of the science wing, he stabbed several classmates. Then he pulled the fire alarm. As the corridor filled with people, the boy moved down the hallway, a knife in each hand, stabbing more students. He turned and raced back up the hall—an administrator remembered him “flailing the knives like he was swimming the backstroke.” One girl later testified, “I could feel that my lip wasn’t attached to my face anymore.” A boy, stabbed in the belly, recalled, “I was gushing blood.”

The students at Franklin Regional, which is seventeen miles east of Pittsburgh, had been trained to lock themselves inside classrooms during a “code red” event. In one room, a home-economics teacher called 911 as she attended to an injured boy. A dispatcher asked where the “patient” had been hurt. “The lower abdomen,” the teacher said. “On the right side.”

“Do you have any way to control the bleeding?” the dispatcher asked.

“I’m putting pressure on it,” the teacher said. She was stanching the blood with paper towels. This was helpful, the dispatcher told her, saying, “If it starts soaking through, I don’t want you to lift it up at all. Find anything else you can to put on top of that.”

The teacher had been applying pressure for about four minutes when the dispatcher said, “We have the actor in custody,” adding, “But I don’t want you to let any of your students leave that room.” As the teacher bore down on the wound, she talked with the injured boy, her voice tense but cheerful. They joked that he could use the experience in a college-application essay. When he predicted that his mother was going to “have a panic attack,” the teacher said, “I think she will.” Then she said, “I never thought I’d have to do this.”

Gracey Evans, a seventeen-year-old junior at the time, remembers that she was walking down the hall with her friend Brett when someone rushed past them “like a black mass.” She didn’t realize that Brett was hurt until he “fell to the ground, withering in pain”—he had been stabbed in the back. In front of her, a boy in a red hoodie grabbed his stomach. A third boy collapsed. The victims took refuge in a nearby science classroom.

Brett’s wound did not look life-threatening, so Gracey dropped to her knees beside the boy in the red hoodie. She raised the hem of his sweatshirt and saw blood pouring out of a clean slit above his waistband. Although she had never witnessed real physical trauma before, she didn’t flinch. Her mother was an orthopedic nurse, and she had seen videos of athletes’ legs broken at grotesque angles.

Gracey recalls that someone handed her a “big wad” of those “terrible” brown paper towels that aren’t very absorbent. She placed them over the gash, interlaced her fingers, and pushed. A dancer, she’d been told that she was stronger than she appeared, and she worried that she might be hurting her classmate, but she kept pressing. The boy suddenly vomited, and part of his liver emerged from the wound. Gracey, nauseated, let go, and blood rushed out again. “I’m so sorry!” she cried, unable to continue. Another student, who happened to be an E.M.T., took over, buying the boy more time.

Twenty-one people had been knifed, several severely, yet everyone survived. (The attacker was later sentenced to a minimum of twenty-three and a half years in prison.) Law-enforcement and health-care professionals in the Pittsburgh area took note of the fortitude and the competence of many bystanders. Listening to a playback of the teacher’s 911 call, they marvelled at her calm and her effectiveness. Brad Orsini, an F.B.I. agent who worked the case, told me, “You’d have thought it was just another day for this woman.” At one point, the teacher had told the boy, “You know what? Sometimes when stuff happens, you go into a different state of mind. You surprise yourself at how you can handle things.”

A category of emergency known as an Intentional Mass Casualty Event is now considered a public-health crisis. In recent years, deadly attacks have occurred at schools, offices, concerts, sporting events, shopping malls, and houses of worship. They have involved guns, knives, trucks, and improvised explosive devices. In March, a gunman killed fifty people at two mosques in Christchurch, New Zealand.

Much attention has been given to the rising frequency of mass shootings in the United States, but equally alarming is their worsening severity. In the attack at Columbine High School, which occurred in 1999, thirteen people were killed and twenty-four were injured. In the 2017 massacre at a music festival in Las Vegas, fifty-eight people were killed and eight hundred and fifty-one were injured. The arsenal of the Las Vegas perpetrator included the AR-15 assault rifle, a weapon that has been used in many rampages. (In fact, he had fourteen of them with him.) Bullets shot from an AR-15 travel at an extremely high velocity; the force of the ammunition can shatter the bones in a human arm merely by grazing it. Last year, Richard Carmona, a former U.S. Surgeon General, said, “More and more, the injuries we’re seeing in the civilian world look like combat casualties.” After the 2018 shooting at Marjory Stoneman Douglas High School, in Parkland, Florida—seventeen killed, seventeen injured—a trauma surgeon opened up a victim to find at least one organ in “shreds,” with “nothing left to repair.”

For victims whose injuries are serious but survivable, rapid treatment is essential. A person can bleed to death in as little as five to eight minutes. Traditionally, during an active-shooter event, paramedics held back until law enforcement secured the area, then rushed in to treat the wounded and evacuate them to hospitals. That approach changed after Columbine. During that event, rescuers, unable to determine if the killers were dead or hiding, didn’t reach some victims for hours. In a second-floor science lab, teachers and students were stranded with a coach, Dave Sanders, who had been shot once in the neck and once in the back. Two Eagle Scouts applied pressure to his wounds, and someone else put a sign in a window—“1 bleeding to death.” Sanders died at the scene; it’s unclear whether he would have survived with quicker or different treatment.

The Las Vegas shooter positioned himself in a sniper’s nest—a hotel window overlooking the festival—and sprayed the crowd with bullets. In this case, it was impossible to stage an orderly transition from a security phase to a medical phase: victims arrived at hospitals in Ubers and pickup trucks, or in the arms of loved ones and strangers.

As public shootings became commonplace, doctors started paying more attention to them. One such doctor was Lenworth Jacobs, the head trauma surgeon at Hartford Hospital, in Connecticut. He’d grown up in Jamaica, where his father was a doctor; when Jacobs was about seven, he and his dad came across an injured bicyclist by the side of the road, and the sight of his father urgently helping a stranger left a lasting impression. Jacobs told me that trauma surgery appealed to him because each case contains a “beginning, middle, and end.” A patient presents with a problem—“a gunshot wound, a stabbing”—which is then resolved, one way or another. When Jacobs wasn’t operating, he devised protocols that would help increase survival rates in the Emergency Department. At Hartford, he founded Life Star, the first helicopter-ambulance service in Connecticut.