In 2001, McQueary told jurors, he walked in on the coach sodomizing a boy of about 10 in a campus locker-room shower.
"It was sexual. It was wrong. It was perverse," he said. "Both of them saw me. I got a good look into both of their eyes."
McQueary's account has drawn intense scrutiny since Sandusky's arrest last year on 52 counts of child sex abuse, in part because it prompted the firing of beloved head football coach Joe Paterno and forced the resignation of university President Graham B. Spanier last year.
Neither official did enough to follow up on McQueary's claims, university trustees said at the time. But their defenders have questioned whether McQueary made clear to Paterno that what he said he saw was sexual in nature.
In court Tuesday, McQueary, 37, forcefully defended his version of events.
Speaking calmly and confidently, he told jurors he was startled by a "skin-on-skin smacking sound" upon entering a campus locker room one night in 2001.
He told of glancing into a mirror and seeing Sandusky standing naked behind a young boy, grasping the boy's stomach and slowly moving his hips.
There was no doubt, McQueary said, that what he witnessed was anal sex.
Taking two more quick looks to make sure, McQueary said he slammed a locker door to alert Sandusky that someone was watching and break up the encounter.
"It was more than my brain could handle," he testified. "I was making decisions on the fly. I picked up the phone and called my father to get advice, because I just saw something ridiculous."
In the following days, McQueary would relate the events to his father, Paterno, and other Penn State officials but said Tuesday he may not have described the scene as graphically as he did for jurors.
He insisted, however, that in each case, those he told "understood it was sexual and wrong."
Sandusky's lawyers seized upon that ambiguity, though, in cross-examination.
Lawyer Karl Rominger peppered McQueary with questions over his failure to immediately alert authorities, mention the words anal sex in early conversations, or initially recall the year in which the alleged incident occurred.
The identity of the boy involved remains unknown, though prosecutors have dubbed him Victim 2. And while the grand jury report that prompted Sandusky's arrest originally said the child was assaulted in 2002, McQueary changed his recollection of the date to 2001 shortly before trial.
At one point, McQueary, appearing frustrated with Rominger's questioning, snapped at the implication he should have done more.
"I didn't physically go remove the young boy from the shower or go punch Jerry out," he said. "When you go through something like this, you're not sure what to do."
Differences of detail also became the focus of defense attacks on Tuesday morning's testimony from the sobbing 18-year-old known as Victim 1. The Inquirer is withholding his name.
In two hours on the witness stand, he never made eye contact with Sandusky and hardly mentioned his name, referring to him instead as "your client" when Sandusky's lawyers were cross-examining him.
Recounting his abuse first for child-services workers, then for a psychologist and eventually for state police, the teen had given differing accounts of the frequency and types of abuse he said he had endured.
Challenged by defense lawyers, he explained: "It's hard enough for me to tell these folks - the jury - this story," he said. "You can't expect a young man to open up and come out with this stuff to new people every single day."
In earlier testimony, the boy said that starting in 2007, Sandusky groped him, kissed him, and forced oral sex upon him during years of sleepovers in the basement of the coach's State College home.
During each encounter, the teen said, he "spaced" and "just kind of blacked out." But despite efforts to resist Sandusky's attentions, he testified of having felt increasingly powerless to escape.
Like many of the coach's accusers, he grew up in a household without a father. He told jurors how his mother was initially excited that Sandusky had chosen to become a positive role model in her son's life.
When he first took his allegations to high school guidance counselors in 2010, they balked.
"They said, 'He [Sandusky] has a heart of gold. He wouldn't do something like that,' " the teen testified, choking back tears. "They didn't believe me."
The day's testimony concluded with yet another tale that underscored how many opportunities adults in the lives of Sandusky's accusers had had to intervene.
Joseph Miller, a wrestling coach at a nearby Clinton County school, testified that in 2007 he walked in on Sandusky and Victim 1 lying on top of each other in a campus gymnasium.
Though Miller told jurors he thought it was odd at the time, Sandusky explained that the two were practicing wrestling moves.
And like the guidance counselor before him, like McQueary with the boy in the shower, Miller told jurors he took no immediate action.
"I thought to myself, 'Well, it's Jerry Sandusky. He's a saint,' " he said. "I didn't think anything more about it, and I left."
Contact Jeremy Roebuck at 267-564-5218 or email@example.com, or follow on Twitter @jeremyrroebuck.
We invite you to comment on this story by clicking here. Comments will be moderated.