Bellefonte, PA (CBS) -- The prosecution called two key witnesses today in the sexual abuse case against former Penn State assistant coach Jerry Sandusky.

An 18-year-old man choked-back tears as he recounted years of abuse this afternoon, and Sandusky's former colleague Mike McQeary took the stand.

Former Assistant football coach Mike McQueary told jurors he saw Jerry Sandusky with a young boy in a shower on the Penn State campus in 2001.

He testified Sandusky was standing behind the boy who appeared to be 10 or 12 propped up against a wall and that he heard skin on skin smacking sounds. McQueary says he closed a locker loudly to indicate he was there.

Earlier today, jurors heard disturbing testimony from an 18-year-old who says Sandusky repeatedly molested him starting when he was a boy.

The teenager testified Sandusky began getting more physical with him during sleepovers, contact that eventually led to oral sex.

He told the hushed courtroom, "I didn't know what to do with all the thoughts running through my head. I just kind of blacked out. I didn't want it to happen. I kind of froze."

Prosecutor: "Did you tell your mom?"
Witness: "No."
Prosecutor: "Did you tell anyone?"
Witness: "No."
Prosecutor: "Did you tell him to stop?"
Witness: "No. I was embarrassed and confused and didn't know what to do."

Eventually the accuser told a school counselor about the years of molestation. But the teen testified that at first the counselor did not believe him saying Sandusky had "a heart of gold."

On tense cross examination Sandusky's defense attorney tried to show the teen changed his story and that his testimony now is more detailed than what he originally told authorities and the grand jury.

Defense attorneys want to prove the accusers are trying to benefit financially from the case.

McQueary told jurors that after he left the locker room he called his dad who told him to leave immediately and come to the house.