(Note: There is graphic testimony about the alleged sexual abuse of a young boy in this post.)

Mike McQueary, a key witness in the case against Jerry Sandusky, testified that he saw the former Penn State assistant football coach engaged in a "clear" "sex act" with a young boy at a campus shower,the Pittsburgh Post-Gazette reports.

McQueary, who at the time was a a 27-year-old graduate assistant and went on to become an assistant coach at the school, was on the stand during the second day of the trial.

As the AP reports, McQueary is the one of the more important witnesses for the prosecution. The AP adds:

"It's the second time McQueary has testified publicly about his 2001 encounter in the Penn State football building. He also testified at a preliminary hearing for two Penn State administrators."McQueary says Sandusky was standing behind a boy who was propped up against a wall. He says the boy appeared to be about 10 to 12 years old."

According to Mark Dent of the Post-Gazette, McQueary testified that after he saw Sandusky and the boy, he went to the locker room.

Vinnie Politan of CNN Headline News reports that McQueary was asked by prosecutors to characterize the sound he heard. McQueary said it was a "slapping sound, skin on skin."

According to the AP, McQueary slammed a door "in an attempt to say 'someone's here, break it up.'" When McQueary went back to the shower, the two had separated.

McQueary also said that he told legendary Penn State coach Joe Paterno what he saw the next morning.

"I told him and I want to make sure I'm clear. I made sure he knew it was sexual and wrong. There was no doubt," the Post-Gazette reports McQueary testified.

So far, the testimony mirrors what McQueary has said before.

Sandusky is accused of sexually abusing 10 boys over the course of 15 years. He has denied the accusations.

Update at 3:43 p.m. ET. Eye Contact:

In its latest write through, the AP reports that McQueary testified he made eye-contact with Sandusky after he and the boy had split.

The AP reports:

"'We looked directly in each other's eyes and at that time I left the locker room,' and went upstairs to his office, he said."'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.'"Copyright 2016 NPR. To see more, visit http://www.npr.org/.