This exchange helps motivate Kotlowitz’s investigation into how people “carry” violence, whether they are perpetrators, victims, witnesses or merely associated with those involved. Kotlowitz interviewed roughly 200 people, and his expansive cast of characters includes social workers, police officers, political officials, a beat reporter and convicted killers, as well as dozens of ordinary people — children and adults whose lives have been shredded by bullets and guns.

Pharoah, he reports in “An American Summer,” had been so traumatized by the shooting he witnessed that, some two decades later, he can’t get it out of his mind. Over lunch in a restaurant, Pharoah hyperventilates and his eyes grow wide with fright as he tells Kotlowitz what happened. “It’s like I’m there,” he says, crouching down as if to take cover. Far from being inured to bloodshed, Pharoah, and others who live in America’s most dangerous neighborhoods, experience the world like war veterans. As Kotlowitz puts it, “The violence is in his bones.”

It’s deeper in Thomas, who grew up in Englewood, a rough, depopulated neighborhood on the South Side. Thomas was unlucky enough to live on 70th Place, a street so treacherous that some call it the “block of death,” and to attend Harper High School, where, during his junior year, 21 students and recent graduates were wounded by gunfire and seven were shot and killed. When he was 10, his 11-year-old friend and neighbor, Nugget, was murdered at her own birthday party, and Thomas saw “her brain matter oozing out of her skull onto her braids.” Months earlier, Thomas’s older brother Leon got shot while playing on the sidewalk. Leon couldn’t move his legs (and never would again), so Thomas sat with him, urging him to hold on until the ambulance arrived.

That was elementary school. After that, Thomas would see a boy shot in the face, a friend shot in the leg, two men who had just been murdered in a car. He joined a gang, or “crew,” called 7-0 for protection, de rigueur for boys on his block, but feared it wouldn’t help. In June 2012, Thomas and his good friend Shakaki were hanging out on the front porch of a house near his when he spotted a hooded boy from a rival crew running toward them, gun in hand. Moments later Shakaki was flat on the porch’s wooden slats, clutching her burning stomach. Eight hours later, she was dead.