Jerry Sandusky built a sophisticated grooming operation, outsourcing to child-care professionals the task of locating vulnerable children—all the while playing the role of lovable goofball. Illustration by A. J. Frackattack; Photograph by Gene J. Puskar / AP

In a 2001 book, “Identifying Child Molesters,” the psychologist Carla van Dam tells the story of a young Canadian elementary-school teacher she calls Jeffrey Clay. Clay taught physical education. He was well liked by his students, and often he asked boys in his class to stay after school, to do homework and help him with chores. One day, just before winter break, three of the boys made a confession to their parents. Mr. Clay had touched them under their pants.

The parents went to the principal. He confronted Clay, who denied everything. The principal knew Clay and was convinced by him. In his mind, what it boiled down to, van Dam writes, “is some wild imaginations and the three boys being really close.”

The parents were at a loss. Mr. Clay was beloved. He had started a popular gym club at the school. He was married and was a role model to the boys. He would come to their after-school games. Could he really have abused them? Perhaps he was just overly physical in the way that young men often are. He had a habit, for example, of grabbing boys in the hallway and pulling them toward him, placing his arms over their shoulders and chest. At the gym club, he would pick boys up and turn them upside down, holding them by the legs. Lots of people—especially gym teachers—like to engage in a little horseplay with young boys. It wasn’t until the allegations about Clay emerged that it occurred to anyone to wonder whether he might have been trying to look down the boys’ shorts.

“We weren’t really prepared to call the police and make it into a police investigation,” one of the mothers told van Dam. “It was an indiscretion, as far as we were concerned at this point. It was all vague: ‘Well, he put his hands down there.’ And, ‘Well, it was inside the pants, but fingers went to here.’ We were all still trying to protect Mr. Clay’s reputation, and the possibility this was all blown up out of proportion and there was a mistake.”

The families then learned that there had been a previous complaint by a child against Clay, and they took their case to the school superintendent. He, too, advised caution. “If allegations do not clearly indicate sexual abuse, a gray area exists,” he wrote to them. “The very act of overt investigation carries with it a charge, a conviction, and a sentence, a situation which is repugnant to fair-minded people.” He was responsible not just to the children but also to the professional integrity of his teachers. What did they have? Just the story of three young boys, and young boys do, after all, have wild imaginations.

Clay was kept on. Two months later, after prodding from a couple of social workers, the parents asked the police to investigate. One of the mothers recalls an officer interviewing her son: “He was gentle, but to the point, and he wanted to be shown exactly where Mr. Clay had touched him.” The three boys named other boys who they said had been subjected to Mr. Clay’s advances. Those boys, however, denied everything. A new, more specific allegation against Clay surfaced. He resigned, and went to see a therapist. But still the prosecutor’s office didn’t feel that it had enough evidence to press charges. And within the school there were teachers who felt that Clay was innocent. “I was running into my colleagues who were saying, ‘Did you know that some rotten parents trumped up these charges against this poor man?’ ” one teacher told van Dam. The teacher added, “Not just one person. Many teachers said this.” A psychologist working at the school thought that the community was in the grip of hysteria. The allegations against Clay, he thought, were simply the result of the fact that he was “young and energetic.” Clay threatened to sue. The parents dropped their case.

Clay was a man repeatedly accused of putting his hands down the pants of young boys. Parents complained. Superiors investigated. And what happened? The school psychologist called him a victim of hysteria.

When monsters roam free, we assume that people in positions of authority ought to be able to catch them if only they did their jobs. But that might be wishful thinking. A pedophile, van Dam’s story of Mr. Clay reminds us, is someone adept not just at preying on children but at confusing, deceiving, and charming the adults responsible for those children—which is something to keep in mind in the case of the scandal at Penn State and the conviction, earlier this year, of the former assistant football coach Jerry Sandusky on child-molestation charges.

Jerry Sandusky grew up in Washington, Pennsylvania. His father headed the local community recreation center, running sports programs for children. The Sanduskys lived upstairs. “Every door I opened, there was a bat, a basketball, a football somewhere,” Sandusky has recounted. “There was constant activity everywhere. My folks touched a lot of kids.” Sandusky’s son E.J. once described his father as “a frustrated playground director.” Sandusky would organize kickball games in the back yard, and, E.J. said, “Dad would get every single kid involved. We had the largest kickball games in the United States, kickball games with forty kids.” Sandusky and his wife, Dottie, adopted six children, and were foster parents to countless more. “They took in so many foster children that even their closest friends could not keep track of them all,” Joe Posnanski writes in “Paterno,” his new biography of Sandusky’s boss, the former Penn State head coach Joe Paterno. “Children constantly surrounded Sandusky, so much so that they became part of his persona.”

Sandusky was a hugger and a grabber and a cutup. “He liked practical jokes and messing around, knocking a guy’s hat off his head, making prank calls, sneaking up behind people to startle them,” Posnanski goes on. People at Penn State thought of him as “a knucklehead.” Much of Sandusky’s 2000 autobiography, “Touched,” is devoted to stories of his antics: the time he smeared charcoal over the handset of his chemistry teacher’s phone, the time he ran afoul of a lifeguard for horseplay with his children in a public pool. Four and a half pages alone are devoted to water-balloon fights that he orchestrated while in college. “Wherever I went, it seemed like trouble was sure to follow,” Sandusky writes. He was a kid at heart. “I live a good part of my life in a make-believe world,” he continues. “I enjoyed pretending as a kid, and I love doing the same as an adult with these kids. Pretending has always been part of me.” There was a time when one of the kids he was mentoring became “cold and unresponsive” to him. It upset him. He writes: