During the second day of the sex abuse trial, McQueary said he heard a "skin-on-skin smacking sound" in a campus locker room one night in 2001. McQueary said witnessing the assault was "more than my brain could handle."
"I immediately became alert and was kind of embarrassed that I was walking in on something," McQueary said.
Sandusky was standing naked in the showers behind a boy, slowly moving his hips, McQueary told the jury. He said he had no doubt he was witnessing anal sex. He estimated the boy to be 10 to 12 years old.
McQueary's testimony is the first eyewitness account of molestation in the trial. His statement to investigators, in addition to those of the accusers, led to Sandusky's arrest on 52 charges of child sex abuse over a span of 15 years.
Legendary football coach Joe Paterno was fired last fall shortly after Sandusky's arrest after it became known that McQueary had told Paterno about the shower incident a decade ago. Two months after the firing, Paterno died of lung cancer at 85.
Earlier Tuesday, the alleged victim who launched the investigation into Sandusky tearfully detailed child abuse and brought some jurors to tears. He told jurors Sandusky was a sexual predator, a pedophile who hand-picked boys from his Second Mile charity.
The now 18-year-old man testified about the first time the former Penn State coach allegedly performed oral sex on him when he was about 12.
"After rubbing, cracking my back and rubbing his hands down back of my shorts, and the blowing on the stomach, he put his mouth on my privates," he said. "I spaced, 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."
In one of their next encounters, he says the acts escalated.
"He looked at me and said something along the lines of, 'It's your turn,' and he made me put my mouth on his privates," he said.
The man, identified as Victim 1, claimed Sandusky forced him into sex acts more than 20 times, and it all happened in Sandusky's basement while his wife was sleeping upstairs.
The man said he was not believed when he first tried to report the alleged abuse to a school guidance counselor.
During cross-examination, defense attorney Joseph Amendola tried to pick through Victim 1's account of events. He said the accuser told child services and police different timelines for when the oral sex began, and pointed out that the accuser didn't mention oral sex until eight months into the investigation - after being told that there were other alleged victims with similar claims.
At one point during the back-and-forth, Victim 1 covered his face, laughing and then crying, saying, "Stop."
When the case worker at Children and Youth Services to whom the case was first referred took the stand, she said Victim 1 had not told her of all the allegations during their first meeting.
Jessica Dershem 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 stomach. Sandusky told her he couldn't recall whether he had ever touched the boy below his waistline.
ABC News and The Associated Press contributed to this report.