Penn State University assistant football on leave, coach Mike McQueary, left, leaves the Centre County Courthouse after testifying in the child sexual abuse trial of former Penn State University assistant football coach Jerry Sandusky in Bellefonte, Pa., Tuesday, June 12, 2012. Sandusky is charged with 52 counts of child sexual abuse involving 10 boys over a period of 15 years.

BELLEFONTE — Jerry Sandusky displayed no visible emotion as more witnesses testified for the prosecution, one an alleged victim who fought back tears describing how the famous assistant football coach at Penn State, who entered his life as a mentor and father figure, turned into a demanding pedophile who stalked him at home and school.

Jurors on Tuesday also heard from Mike McQueary who in much anticipated testimony stood firmly by his claim that he saw Sandusky, naked in a team shower late one Friday night, in a position that he remains convinced meant Sandusky was sodomizing a child of about 10.

So-called “Victim 1,” the boy whose mother alerted authorities to suspicions of abuse in 2008, launching the investigation that eventually led to the 52 criminal charges Sandusky faces, struggled to retain his composure while recounting the humiliation and fear that accompanied the alleged abuse at Sandusky’s hands.

The boy, now 18 and a recent high school graduate, said sleepovers in a bedroom in the basement of Sandusky’s State College home escalated into alleged kissing, fondling and oral sex. Eventually, he testified, Sandusky said it was his turn to reciprocate the act of his alleged molester. He said he could not resist.

“I don’t know how to explain it, I froze,” he said. “My mind is telling me to move but I couldn’t do it, I couldn’t move.”

During his testimony, McQueary said, in a voice alternating between calm and forcefulness, he walked into a university athletics locker room more than a decade ago to the sound of a “skin-on-skin smacking sound.” In a mirror, he said, he could see in the showers Sandusky, standing behind a boy who was “propped up against a wall.”

McQueary, who was then a graduate coaching assistant, said that while he couldn’t say with “1,000 percent” certainty, he is convinced Sandusky “was having sex with (him), yes.”

“The showers are running. And he is right up against his back with his front. The boy’s hands are up against the wall,” he said, and they are in “the closest proximity that I think you can be in.”

He said “the defendant’s midsection was moving” subtly.

McQueary 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, eventually reporting it to then-head football coach Joe Paterno and telling his story to two university officials. It would be more than a decade before he shared what he saw with police investigators.

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 the administrators charged with failing to report the shower episode to authorities. One difference: He said it took place in 2001 instead of 2002, and that and other discrepancies were the focus of cross-examination.