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As many as six assistant coaches at Penn State allegedly witnessed "inappropriate behavior" between Jerry Sandusky and boys, stretching as far back as the 1970s, NBC News has learned.

It is unclear if any of the men made a report to higher-ups at Penn State before the sex-abuse scandal erupted in 2011.

The information, which comes from court documents and multiple sources with direct knowledge of legal proceedings, raises new questions about how long the abuse went on, why no one stopped it and whether there could be even more victims than previously known.

Sandusky — who worked in the football program at Penn State under legendary head coach Joe Paterno for three decades — is serving 30 to 60 years in prison after being convicted of molesting 10 boys he met through a charity starting in 1994.

But sources told NBC News that one former Penn State assistant coach witnessed an incident in the late 1970s. Three other coaches — who have gone on to work in the NFL and at Division I colleges — allegedly saw inappropriate conduct between Sandusky and boys in the early and mid-1990s.

“You won’t believe what I just saw,” one of those three coaches blurted out after bursting into a room filled with Penn State football staff, according to sources who spoke to a person who was in that room.

A lawyer for one of the three ‘90s coaches denied his client had seen anything. A second coach declined to comment. A third could not be reached, and the name of the fourth was not disclosed to NBC News.

Bolstering the sources’ account, Sandusky’s adopted son, Matt, who says he also was molested, told NBC News that investigators informed him a football program employee witnessed what he believed to be a sex act between Matt and Sandusky in a locker room in the early 1990s.

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Former Penn State University assistant football coach Jerry Sandusky, center, leaves the Centre County Courthouse after a hearing of arguments on his request for an evidentiary hearing in Bellefonte, Pa., Monday, May 2, 2016. Gene J. Puskar / AP

In addition to the four assistants detailed by sources, court papers made public this week point to two more coaches who allegedly witnessed what was described as “inappropriate” or “sexual” contact between Sandusky and children in 1987 and 1988.

Those same documents also revealed that a child allegedly told Joe Paterno he was molested by Sandusky in 1976 — 25 years earlier than Paterno acknowledged hearing about Sandusky in a shower with a child.

Those papers are tied to a legal dispute between Penn State and an insurance company over who should pay the university’s share of $60 million in settlements to 26 victims, and the court was not required to verify the allegations.

Asked about the four coaches detailed by NBC's sources, Penn State released a written statement.

"The university is facing and has faced a number of litigation matters and claims related to the Sandusky events. Allegations of various kinds have been made, and will likely continue to be made," it said.

"The university does not speculate publicly or hypothesize about individual allegations. These are sensitive matters, and we want to be respectful of the rights of all individuals involved. It would be inappropriate to do otherwise."

School President Eric Barron issued a letter to the university community on Sunday in which he labeled the allegations about Paterno and assistant coaches "innuendo."

"The allegations related to Penn State are simply not established fact," he said.

Read: Penn State President's Letter to University

Sandusky, 72, recently petitioned for a new trial.

Paterno died of lung cancer in 2012, months after being fired as head coach amid allegations he knew about Sandusky’s abuse and failed to act.

His family is hitting back at the claim that he knew about Sandusky’s crimes back in 1976.

"Because of a single sentence in a court record of an insurance case, Joe Paterno's reputation has once again been smeared with an unsubstantiated, forty year old allegation," the family said in a written statement.

"Over the past four and a half years, numerous allegations that were taken as fact when they were initially communicated have been proven false. It is in this context that the latest claim should be viewed."

Penn State said it could not comment on the allegation that a child told Paterno he had been abused.

“The university has no records from the time to help evaluate the claims. More importantly, Coach Paterno is not here to defend himself. Penn State does not intend to comment further, out of concern for privacy, and due to the strict confidentiality commitments that govern our various settlement agreements.”

This story has been updated to reflect Penn State's latest statement.