The coach who says Jerry Sandusky molested a boy in a Penn State locker room in 2002 has spoken about the incident for the first time in public.
Mike McQueary said "it was very clear that it looked like there was intercourse" and that Mr Sandusky held his hands around the boy's waist.
Mr Sandusky, 63, denies 52 counts of child sexual abuse of 10 victims.
Mr McQueary, 37, spoke at a hearing in the perjury case against two officials at the university.
The officials, Tim Curley and Gary Schultz, are accused of lying to the grand jury that investigated the sexual abuse charges against Mr Sandusky.
A judge ruled after the hearing that there was enough evidence to try both men for perjury.
On Tuesday, Mr Sandusky's defence lawyers waived their right to a similar preliminary hearing, saying having the allegations repeated before a trial would be "the worst of all worlds".
Eleven witnesses, including accusers, had been poised to give evidence in that hearing.
Mr McQueary testified on Friday at a court in Harrisburg, Pennsylvania, that because he could not see Mr Sandusky's genitals, he was not totally sure penetration had occurred. But he characterised what he witnessed as "extremely sexual".
Mr Curley and Mr Schultz told the grand jury they could not recall Mr McQueary telling them he had seen Mr Sandusky raping a boy - only something like wrestling.
Prosecutors say Mr Sandusky continued to abuse boys for six more years.
During his hearing testimony, Mr McQueary said he met the two university officials about 10 days after the incident.
"I told them that I saw Jerry in the showers with a young boy and that what I had seen was extremely sexual and over the lines and it was wrong," he said.
"I would have described that it was extremely sexual and I thought that some kind of intercourse was going on."
Mr McQueary said he saw Mr Sandusky's hands around the waist of the boy, who Mr McQueary estimated to be 10 or 12 years old, and he heard skin-on-skin slapping sounds.
A graduate assistant with the football team at the time, Mr McQueary said he did not go to local authorities because Mr Schultz oversaw the campus police.
"In my mind it was like speaking to a [district attorney]. It was someone who police reported to and would know what to do with it," he said.