Mike McQueary also said in an e-mail obtained by several media outlets he had "discussions with police and with the official at the university in charge of police" after stopping the alleged sexual assault.
A grand jury report states that in 2002, while a graduate assistant, McQueary witnessed Sandusky allegedly engaged in a sexual act with a boy in the Penn State locker room's showers.
It also states McQueary left after seeing the alleged attack and later reported it to head Coach Joe Paterno. But no mention was made about an attempt to stop the attack.
"I did stop it, not physically, but made sure it was stopped when I left that locker room," McQueary said in the e-mail obtained by The (Allentown, Pa.) Morning Call.
McQueary also wrote: "No one can imagine my thoughts or wants to be in my shoes for those 30-45 seconds. Trust me."
The wide receivers coach has been placed on indefinite administrative after threats were made against him.
Meanwhile Tuesday, police said they're trying to confirm allegations by more suspected sex abuse victims of Sandusky.
Officials said up to 10 additional suspected victims have come forward since Sandusky was arrested in State College, Pa., Nov. 5 on 40 counts of sexually abusing young boys, The New York Times reported Monday.
The case led to the ouster of Paterno, Penn State President Graham B. Spanier and several other university officials.
Sandusky told NBC News in a telephone interview that aired Monday he showered with boys and played with them but didn't sexually abuse them.
NBC said Sandusky acknowledged he took showers and "horsed around" with young boys. But when asked directly whether he is a pedophile, Sandusky answered, "No."
"I could say that I have done some of those things," he said. "I have horsed around with kids. I have showered after workouts. I have hugged them and I have touched their legs without intent of sexual contact."
The 67-year-old ex-defensive coordinator for one of the nation's top major college football programs faces charges that he sexually abused up to eight boys at his home, Penn State locker rooms and hotels dating back to 1994.
While he said, "I shouldn't have showered with those kids," he also said, "I say that I am innocent of those charges."
He is free on $100,000 bond.
Sandusky's attorney, Joseph Amendola, told NBC he expects at least some of the alleged victims will deny being molested.
"We expect we're going to have a number of kids, now how many of those so-called eight kids we're not sure, but we anticipate we're going to have at least several of those kids come forward and say this never happened," Amendola said. "In fact, one of the toughest allegations ... what McQueary said he saw, we have information that that child said that never happened."
On Sunday, Jack Raykovitz resigned as chief executive of the Second Mile foundation, a charity for needy children Sandusky started in 1977. The Pennsylvania attorney general has said Sandusky used the foundation to prey on young boys and that he met eight boys mentioned in the grand jury report through the foundation. Sandusky resigned from the foundation's day-to-day activities last year.