Every American can be his own policeman; the country has nearly as many guns as it has people. Photograph by Christopher Griffith

Just after seven-thirty on the morning of February 27th, a seventeen-year-old boy named T. J. Lane walked into the cafeteria at Chardon High School, about thirty miles outside Cleveland. It was a Monday, and the cafeteria was filled with kids, some eating breakfast, some waiting for buses to drive them to programs at other schools, some packing up for gym class. Lane sat down at an empty table, reached into a bag, and pulled out a .22-calibre pistol. He stood up, raised the gun, and fired. He said not a word.

Russell King, a seventeen-year-old junior, was sitting at a table with another junior, Nate Mueller. King, shot in the head, fell face first onto the table, a pool of blood forming. A bullet grazed Mueller’s ear. “I could see the flame at the end of the gun,” Mueller said later. Daniel Parmertor, a sixteen-year-old snowboarder, was shot in the head. Someone screamed “Duck!” Demetrius Hewlin, sixteen, was also shot in the head, and slid under the table. Joy Rickers, a senior, tried to run; Lane shot her as she fled. Nickolas Walczak, shot in his neck, arm, back, and face, fell to the floor. He began crawling toward the door.

Ever since the shootings at Columbine High School, in a Denver suburb, in 1999, American schools have been preparing for gunmen. Chardon started holding drills in 2007, after the Virginia Tech massacre, when twenty-three-year-old Seung-Hui Cho, a college senior, shot fifty-seven people in Blacksburg.

At Chardon High School, kids ran through the halls screaming “Lockdown!” Some of them hid in the teachers’ lounge; they barricaded the door with a piano. Someone got on the school’s public-address system and gave instructions, but everyone knew what to do. Students ran into classrooms and dived under desks; teachers locked the doors and shut off the lights. Joseph Ricci, a math teacher, heard Walczak, who was still crawling, groaning in the hallway. Ricci opened the door and pulled the boy inside. No one knew if the shooter had more guns, or more rounds. Huddled under desks, students called 911 and texted their parents. One tapped out, “Prayforus.”

From the cafeteria, Frank Hall, the assistant football coach, chased Lane out of the building, and he ran off into the woods.

Moments later, four ambulances arrived. E.M.T.s raced Rickers and Walczak to Chardon’s Hillcrest Hospital. Hewlin, Parmertor, and King were flown by helicopter to a trauma center at MetroHealth Medical Center, in Cleveland. By eight-thirty, the high school had been evacuated.

At a quarter to nine, police officers with dogs captured Lane, about a mile from the school.

“I hate to say it, but we trained for exactly this type of thing, a school emergency of this type,” Dan McClelland, the county sheriff, said.

Danny Parmertor died that afternoon. That evening, St. Mary’s Church opened its doors, and the people of Chardon sank to their knees and keened. At the town square, students gathered to hold a vigil. As night fell, they lit candles. Drew Gittins, sixteen, played a Black Eyed Peas song on his guitar. “People killin’, people dyin’,” he sang. “People got me, got me questionin’, Where is the love?”

Russell King had been too badly wounded. A little after midnight, doctors said that they couldn’t save him.

There are nearly three hundred million privately owned firearms in the United States: a hundred and six million handguns, a hundred and five million rifles, and eighty-three million shotguns. That works out to about one gun for every American. The gun that T. J. Lane brought to Chardon High School belonged to his uncle, who had bought it in 2010, at a gun shop. Both of Lane’s parents had been arrested on charges of domestic violence over the years. Lane found the gun in his grandfather’s barn.

The United States is the country with the highest rate of civilian gun ownership in the world. (The second highest is Yemen, where the rate is nevertheless only half that of the U.S.) No civilian population is more powerfully armed. Most Americans do not, however, own guns, because three-quarters of people with guns own two or more. According to the General Social Survey, conducted by the National Policy Opinion Center at the University of Chicago, the prevalence of gun ownership has declined steadily in the past few decades. In 1973, there were guns in roughly one in two households in the United States; in 2010, one in three. In 1980, nearly one in three Americans owned a gun; in 2010, that figure had dropped to one in five.

Men are far more likely to own guns than women are, but the rate of gun ownership among men fell from one in two in 1980 to one in three in 2010, while, in that same stretch of time, the rate among women remained one in ten. What may have held that rate steady in an age of decline was the aggressive marketing of handguns to women for self-defense, which is how a great many guns are marketed. Gun ownership is higher among whites than among blacks, higher in the country than in the city, and higher among older people than among younger people. One reason that gun ownership is declining, nationwide, might be that high-school shooting clubs and rifle ranges at summer camps are no longer common.

Although rates of gun ownership, like rates of violent crime, are falling, the power of the gun lobby is not. Since 1980, forty-four states have passed some form of law that allows gun owners to carry concealed weapons outside their homes for personal protection. (Five additional states had these laws before 1980. Illinois is the sole holdout.) A federal ban on the possession, transfer, or manufacture of semiautomatic assault weapons, passed in 1994, was allowed to expire in 2004. In 2005, Florida passed the Stand Your Ground law, an extension of the so-called castle doctrine, exonerating from prosecution citizens who use deadly force when confronted by an assailant, even if they could have retreated safely; Stand Your Ground laws expand that protection outside the home to any place that an individual “has a right to be.” Twenty-four states have passed similar laws.

The day before T. J. Lane shot five high-school students in Ohio, another high-school student was shot in Florida. The Orlando Sentinel ran a three-paragraph story. On February 26th, seventeen-year-old Trayvon Martin left a house in a town outside Orlando and walked to a store. He was seen by a twenty-eight-year-old man named George Zimmerman, who called 911 to report that Martin, who was black, was “a real suspicious guy.” Zimmerman got out of his truck. Zimmerman was carrying a 9-mm. pistol; Martin was unarmed. What happened next has not been established, and is much disputed. Zimmerman told the police that Martin attacked him. Martin’s family has said that the boy, heard over a cell phone, begged for his life.

Zimmerman shot Martin in the chest. Martin did not survive. Zimmerman was not charged. Outside Orlando, the story was not reported.

The day after the shooting in Ohio, I went to a firing range. I’d signed up for a lesson the week before. Once, when I was in Air Force R.O.T.C. for a year, I spent an afternoon studying how to defeat a sniper, but I’d never held a gun before.