Jerry Sandusky was standing naked in the showers behind a boy, slowly moving his hips, Mike McQueary told the jury.
McQueary, one of the star witnesses in the child sexual abuse case against Sandusky, said he had no doubt he was witnessing anal sex. He testified that he slammed his locker shut loudly as if to say, "Someone's here! Break it up!"
Then, he said, he went upstairs to his office to try to make sense of what he had seen.
Sandusky, 68, is on trial on charges he molested 10 boys over a 15-year period. Authorities say he abused them in hotels, at his home and inside the football team's quarters. The former assistant coach and founder of an acclaimed youth charity has denied the allegations.
Paterno was fired last fall, shortly after Sandusky's arrest, when it became known that McQueary had told the head football coach about the shower episode a decade ago. Two months after his dismissal, Paterno died of lung cancer at 85.
McQueary was composed during his testimony, and when asked if he knew Sandusky, he looked right at him with a sharp glance that Sandusky returned.
McQueary's account differed little from the one he gave in December at a preliminary hearing for two Penn State administrators charged with failing to report the shower episode to authorities. One difference: He said it took place in 2001 instead of 2002.
Sandusky lawyer Karl Rominger pressed McQueary during cross-examination about discrepancies in his estimate of the boy's age.
McQueary replied: "If (you) want to argue about 9, 10, 11, 12 ... the fact is he had sex with a minor, a boy."
Testifying on Day 2 of Sandusky's trial, McQueary said that he went to the football team building one night and walked into the support staff locker room to put away a pair of new sneakers. As he entered the locker-room, he said, he heard a noise.
"Very much skin-on-skin smacking sound," he said. "I immediately became alert and was kind of embarrassed that I was walking in on something."
He said that he glanced over his shoulder at a mirror at a 45-degree angle and saw Sandusky "standing behind a boy who was propped up against a wall." He estimated the boy to be 10 to 12 years old. He said that the boy's hands were up on the wall and "the defendant's midsection was moving" subtly.
"The glance would have taken only one or two seconds. I immediately turned back to my locker to make sure I saw what I saw," he said.
He said he wasn't sure whether Sandusky saw him. After slamming his locker to make some noise, he left.
"It was more than my brain could handle," he said. "I was making decisions on the fly. I picked up the phone and called my father to get advice from the person I trusted most in my life, because I just saw something ridiculous."
He said he was extremely vague with his father, who told him to leave immediately.
McQueary said he went to Paterno's house the next morning and relayed what he had seen, but did not describe the act explicitly out of respect for the coach and his own embarrassment.
He said Penn State administrator Tim Curley called him a week later, and McQueary met with him and another school official, Gary Schultz. They "just listened to what I had said," McQueary testified. A week or two later, he said, Curley called him to say they had looked into it.
McQueary, 37, was a graduate coaching assistant at the time and later became an assistant coach. He has been on paid leave since the scandal erupted.
The identity of the boy who was said to have been in the showers is a mystery to prosecutors. In fact, two of the 10 alleged victims have yet to be located or even identified.
Earlier Tuesday, the teenager who triggered the grand jury investigation that rocked Penn State became the second of the alleged victims to take the stand.
Choking back tears, he said that Sandusky kissed him, fondled him and engaged in oral sex with him during numerous sleepovers in the basement of Sandusky's home while the coach's wife was upstairs.
The accuser, labelled Victim No. 1 by a grand jury, said he eventually confided in a school district guidance counsellor that Sandusky was molesting him, only to be told by school officials: "He has a heart of gold, and he wouldn't do something like that."
"So they didn't believe me," the teenager said.
School officials ultimately referred the case to the county's child-welfare agency, which found his account credible.
Now 18, he told the jury about an early encounter with Sandusky that escalated to oral sex.
"I spaced," he said. "I didn't know what to do with all the thoughts running through my head, I just kind of blacked out and didn't want it to happen. I froze."
He said he stayed quiet about the abuse, in part because his mother thought Sandusky was a positive influence, but he began trying to distance himself from Sandusky.
Sandusky got angry with him because they had drifted apart, and things escalated into an argument between the boy's mother and Sandusky, the teenager said.
"I got extremely, extremely scared," he said.
Eventually the teen asked his mother if there was a website used to track sex offenders because he wanted to see if Sandusky was on it. That led to a meeting with the guidance counsellor.
Jessica Dershem, a child-welfare caseworker who spoke to Sandusky, testified that the coach denied having sexual contact with the boy but did acknowledge lying on top of him and blowing "raspberries" on the boy's belly. Dershem said Sandusky told her he couldn't recall whether he had ever touched the boy below the waist.
During cross-examination, Sandusky lawyer Joe Amendola asked the teen whether he had financial motives for bringing his accusations.
"All I know is I'm here to tell the truth about what happened to me, just like everybody else," he replied.
Amendola pressed the accuser about his initial statements to a counsellor and later the grand jury that were less detailed than later testimony. The teen, who graduated from high school last week, responded that it was an embarrassing subject.
"I don't believe anybody would want to talk about it," he said.
Sandusky didn't visibly react to the teen's account and looked straight ahead during his testimony.