The charges came to light after Sandusky began grooming a boy of 11 or 12 with expensive gifts and trips to sports events after meeting him through his Second Mile Organization. Then he began physical contact during overnight stays at his home.
That triggered an investigation, which culminated in charges, being brought against not just him but also Athletic director Tim Curley and the school's vice president for finance and business Gary Schultz, who cops say covered for the highly thought of coach.
Attorney General Linda Kelly said that seven years before that, in 2002, a graduate assistant saw Sandusky sexually assault a naked boy, estimated to be about 10 years old, in the locker room of the Lasch Football Building on campus.
She called him "a sexual predator who used his position within the university and community to repeatedly prey on young boys."
She added: "It is also a case about high-ranking university officials who allegedly failed to report the sexual assault of a young boy after the information was brought to their attention, and later made false statements to a grand jury.
"Despite a powerful eyewitness statement about the sexual assault of a child, this incident was not reported to any law enforcement or child protective agency, as required by Pennsylvania law."
Curley and Schultz are expected to turn themselves in on Monday.
Sandusky meanwhile has been charged with seven counts of involuntary deviate sexual intercourse; eight counts of corruption of minors, eight counts of endangering the welfare of a child, seven counts of indecent assault and other offences.
He denies the charges.