Mike McQueary, the graduate assistant who a grand jury report said saw Jerry Sandusky allegedly sodomizing a boy in the locker room, said in the email that he stopped the act.
According to court documents, McQueary watched as Sandusky coach raped a 10-year-old boy in the shower in 2002. McQueary then told head coach Joe Paterno, but he hasn't spoken publicly about it until now.
McQueary said in an interview with CBS that his emotions are all over the place right now and that's he's feeling "shaken." He poured out some of those emotions in a private email to a friend, which was obtained by the Associated Press.
He wrote, "I did stop it, not physically ... but made sure it was stopped when I left that locker room ... I did have discussions with police and with the official at the university in charge of police .... no one can imagine my thoughts or wants to be in my shoes for those 30-45 seconds ... trust me."
Sandusky has been charged with 40 criminal counts for alleged sexual abuse against eight boys over 15 years.
Sandusky claims that he's innocent. He told NBC's Bob Costas there was nothing sexual about a man in his 50s showering with pre-teens.
"I have done some of those things. I have horsed around with kids. I have showered after workouts. I have hugged them. I have touched their legs," said Sandusky.
That interview may hurt Sandusky later in court.
"Sandusky basically incriminated himself anyway. Naked showers, horseplay - what the hell is inadvertent touching of a leg? How does that happen and what does that mean?" said sports journalist Buzz Bissinger.
The mother of one of the alleged victims responded Tuesday night on "Nightline."
"It sickened me that he would be on TV trying to downplay his charges. I think he made himself look more guilty," said the mother.
It's hard to know if McQueary actually filed a police report, because, back in the late 1990s, Penn State successfully lobbied for a special exemption to the state's public records act. The school is not required to disclose many of the basic records almost all government agencies in the state have to.
McQueary is a former player and current assistant coach who was placed on indefinite paid leave last week after school officials said he had received threats.
The scandal has resulted in the firing of coach Paterno, the ousting of president Graham Spanier and charges of perjury against the athletic director and a former senior vice president.
Sandusky's next court date is Dec. 7, when he is due for a preliminary hearing in which a judge would determine if there's enough evidence for prosecutors to move forward with the case.
See photos of Penn State students reacting to the firing of Joe Paterno.
The Associated Press contributed to this report.