Sexual abuse was so rampant that it created a “private hell” for some students at an elite prep school in Rhode Island in the 1970s and ’80s, investigators reported on Thursday, describing an atmosphere of terror in which at least 61 students were victimized and some staff members committed assaults for years before being forced out.

The investigation found that at least 51 students were abused by employees of St. George’s School, a prestigious private boarding and day school near Newport, and at least 10 others by fellow students — and that the true numbers are probably significantly higher. The inquiry came after years of pressure from victims who said that St. George’s had refused to squarely acknowledge the extent of the problems.

The report — with pages of vivid, painful detail from accusers — follows other revelations about sexual abuse at several prominent prep schools in the same area, but none have matched the sheer scale and pervasiveness of the misconduct discovered at St. George’s.

More victims will come forward, and “when the sun is set on this case, it could be the largest school sex abuse case in history,” said Roderick MacLeish, a lawyer and St. George’s alumnus who has worked with abuse survivors in pressuring the school to conduct the investigation. “It’s in the thousands of years of suffering this caused.”