It's the allegation that Penn State acknowledges ended Joe Paterno's distinguished football coaching career and spawned criminal charges against two school officials.
But the only person who says he saw it happen is another former assistant. Prosecutors don't know who the boy is, while Sandusky says he believes he does know, and that the now-grown man, referred to in court papers only as Victim 2, could exonerate him.
Even the timing of the allegation is in question, as is the age of the boy a decade ago.
All the conflicting information presents tough challenges for prosecutors - not just at the sex abuse trial beginning in mid-May, at which the defense does not plan to call the man, but also in the court of public opinion.
"I'm not trying to make light of the situation, but how can you say it's murder if there's no body?" said 1982 Penn State alumna Wendy Silverwood, a saleswoman from West Chester, Pa., who said she believes Paterno was not given a fair shake. "If you don't know who the victim is, and you can't identify and speak with them, how can you bring charges?"
As recently as Thursday, Sandusky's lawyer argued in court filings that there wasn't enough evidence to support the charges relating to Victim 2. Sandusky, 68, faces 52 criminal counts involving 10 boys dating to the late 1990s and denies all the allegations.
The lawyer, Joe Amendola, told The Associated Press that a young man contacted him after Sandusky's November arrest to say he believed he might be the person referred to as Victim 2. After meeting with him, along with his mother and adult brother, Amendola was left with doubts.
"I wasn't sure he was," Amendola said. "I'm still not sure. I haven't been able to verify it. Jerry's very sure."
Amendola said that the young man told him Sandusky had not abused him, but that he later obtained a lawyer and cut off contact. Amendola does not plan to subpoena the young man and declined to identify him or his lawyer.
"I don't want to put someone on the stand who might say something completely different," Amendola said. "And quite honestly, now that he's got a lawyer, he might say something different."
Records supplied by prosecutors indicate some purported victims have changed their stories, the lawyer said.
"Several of the kids, who are so-called victims now, initially said nothing happened," Amendola said. "And now they're victims."
Mike McQueary, who in 2002 was a graduate assistant for the football team, testified at the December preliminary hearing that he saw Sandusky and the boy, both naked, after hearing skin-on-skin slapping sounds. He called it "extremely sexual" and "some kind of intercourse."
McQueary said he reported what he saw in the locker room shower to Paterno and Penn State administrators Tim Curley and Gary Schultz. Exactly what he saw and what he told them are both certain to be hotly contested at Sandusky's trial, as well as at the pending trials of Curley and Schultz on charges they failed to properly report suspected abuse.
Penn State trustees have said Paterno's lack of follow-up after McQueary's report was behind their decision to summarily fire him in November, before the end of the football season. The dismissal of Paterno, who died in January of lung cancer, has rankled alumni and other supporters.
Even the year of the shower incident is in dispute.
Sandusky's lawyer said that his client is convinced it was in 2001, not 2002 as the prosecution has said, and that Sandusky offered to help Curley find the boy when the administrator asked him about McQueary's complaint. Amendola said Curley never mentioned McQueary's name, and Sandusky does not recall seeing McQueary.
Sandusky told Curley at the time that he knew the young man in question but they had been only horsing around, sliding around inside the wet shower, the lawyer said.
Sandusky said back then that "if Tim Curley wanted to verify that, Jerry offered to give him the name and number of the young kid," Amendola said. "Curley seemed satisfied with that," he said, and did not get the boy's name from Sandusky.
"The reason he remembers is that Jerry contacted him after that shower situation and said someone from Penn State may contact him," Amendola said. "He said nothing sexual occurred at that time between him and Jerry. In fact, the mother said Jerry was a godsend to the family."
Caroline Roberto, a lawyer for Curley, said only that Curley acted appropriately judging by what he knew at the time. Curley and Schultz have both denied the allegations and are asking a judge to dismiss the charges.
Prosecutors said this month in a court filing that they still did not know the boy's identity, raising questions about whether the man's lawyer contacted the attorney general's office.
Victim 2 is not the only mystery in the case.
There is a second alleged victim who has not been identified by investigators and is being called Victim 8. A grand jury report alleged he was seen by Penn State janitor Jim Calhoun in fall 2000 in athletic department showers with Sandusky, pinned against the wall as Sandusky performed oral sex on him.
Calhoun told another janitor and a supervisor what he saw, the grand jury said, but as of November suffered from dementia and was described as incompetent to testify.
Amendola considers the charges related to Victim 2 and Victim 8 the weakest part of the government's case.
"I think that creates a problem for the commonwealth," he said. "And the commonsensical reaction would be, if the stuff really occurred, why didn't they come forward and say, 'I'm the guy'?"
State prosecutors, who need to be able to prove the ages of victims, declined to discuss the issue of the two identities.
"This case has been the result of an extensive investigation and an extensive grand jury investigation," said Nils Frederiksen, a spokesman for the attorney general's office. "We have a high degree of confidence in the case, but we're not going to discuss the strategy of how our prosecutors plan to present the case in court. It's just not appropriate."
To establish the age of anonymous children in child pornography cases, prosecutors sometimes have pediatric specialists apply standard measures of development, a technique that might be used in the Sandusky case.
"It's a little bit unusual to prove a child rape case this way, but it's also unusual to have an eyewitness to child rape," said Christopher Mallios, a former Philadelphia deputy district attorney who helps train police and prosecutors in sexual violence cases.
Jurors may wonder why the young men have not stepped forward, despite the detailed reports of abuse and the extensive publicity surrounding Sandusky's arrest. But that would not be surprising, Mallios said, given what he saw during investigations in Philadelphia of abuse allegations against Roman Catholic clergy members.
"A lot of the victims did not tell anyone about what had happened to them until well into their 50s," he said. "They just couldn't talk about it. Even when the investigators were able to piece together their identities by talking to other victims, some just wouldn't talk about it."