The brother of Pope Emeritus Benedict XVI established a "reign of fear" while head of boys' choir at one of Germany's most famous Roman Catholic schools, where more than 500 pupils were physically and sexually abused between 1945 and 2015.
A report published this week accused 49 members of abusing 547 young boys who sang in the Regensburger Domspatzen choir over a period of 60 years.
Benedict's older brother, Georg Ratzinger, was the choirmaster between 1964 and 1994.
His successor, Roland Büchner, told German newspaper Die Zeit that Ratzinger was an "impulsive, fanatical and merciless" teacher when "imposing his idea of musical discipline".
He said that the young boys lived in a "reign of terror" and that many were physically abused.
"It wasn't just slaps, but genuine physical abuse. People were raging, there were injuries," Büchner said. "Afterwards, he could be the gentlest person in the world. Some pupils saw him as a model, others feared him as someone who would beat them."
Ratzinger, now 93, is the older brother of 90-year-old Joseph Aloisius Ratzinger, who became Pope Benedict XVI in 2005 until his papacy ended in 2013.
Regensburger Domspatzen choir was thrust into the spotlight this week when investigator Ulrich Weber presented a 440-page report, detailing how pupils at the school were subjected to abuse for decades.
According to the report, boys were slapped in the face so hard that the marks could still be seen the following day, whipped with wooden sticks and subjected to severe beatings. At least 67 children are said to have suffered sexual abuse, including rape.
"Victims described the institution as a prison, hell and a concentration camp," Weber told a news conference. "Many of them called the time there as the worst of their lives, which was marked by violence, fear and helplessness."
Ratzinger denies any knowledge of sexual abuse and has justified the physical violence by claiming that slapping pupils was normal at the time. But Weber accuses the choirmaster of "looking the other away" and a "lack of intervention despite knowledge".
He also criticised Cardinal Gerhard Ludwig Müller, who was the Bishop of Regensburg at the time, for failing to confront the issue. Müller was recently removed as head of the Congregation of the Doctrine of the Faith by Pope Francis.