In an emotional press conference on Tuesday, Pennsylvania Attorney General Josh Shapiro unveiled a more than 800-page grand jury report that chronicles seven decades of sexual abuse of children by priests in six Catholic dioceses and the “systematic cover up by senior church officials in Pennsylvania and at the Vatican.”

Mr. Shapiro said that his office’s two-year investigation identified 301 priests who abused children and more than 1,000 victims. He said members of the grand jury told him they believe the number of victims is much larger.

He described in detail many of the acts of abuse, saying church leaders had employed euphemisms in the past to cover up what was happening.

“Church officials routinely and purposely described the abuse as horseplay and wrestling and inappropriate contact. It was none of those things,” Mr. Shapiro said. “It was child sexual abuse, including rape, committed by grown men, priests, against children.”

Mr. Shapiro said that his office’s two-year investigation identified 301 priests who abused children and more than 1,000 victims.

He slammed bishops and other church leaders for thwarting inquiries, protecting priests and acting hostile toward victims.

“The cover-up was sophisticated. And all the while, shockingly, church leadership kept records of the abuse and the cover-up. These documents, from the dioceses’ own ‘Secret Archives,’ formed the backbone of this investigation,” Mr. Shapiro said at the news conference in Harrisburg.

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“Above all else, they protected their institution, at all cost,” Mr. Shapiro said.

Mr. Shapiro lamented that some church leaders cited in the report for mishandling abuse claims have been promoted, including Cardinal Donald Wuerl, the archbishop of Washington who headed the Diocese of Pittsburgh from 1988 to 2006.

“Bishop Wuerl,” he said, “is now Cardinal Wuerl.”

The cardinal released a statement on Tuesday, in which he called the sexual abuse of children “a terrible tragedy” and defended his own record in Pittsburgh.

“As I have made clear throughout my more than 30 years as a bishop, the sexual abuse of children by some members of the Catholic Church is a terrible tragedy, and the Church can never express enough our deep sorrow and contrition for the abuse, and for the failure to respond promptly and completely,” Cardinal Wuerl said.

“While I understand this Report may be critical of some of my actions, I believe the Report confirms that I acted with diligence, with concern for the victims and to prevent future acts of abuse,” he continued. “I sincerely hope that a just assessment of my actions, past and present, and my continuing commitment to the protection of children will dispel any notions otherwise made by this report.”

The head of the U.S. Conference of Catholic Bishops, Cardinal Daniel DiNardo, also condemned sexual abuse and said that U.S. bishops “are shamed by and sorry for the sins and omissions by Catholic priests and Catholic bishops.”

But he said policy changes bishops made in 2002 have made the church a safe place for children and holds priests accountable for abuse.

The report acknowledges that most of the abuse cases it documents occurred “before the early 2000’s” and it says it invited bishops to explain how their dioceses have changed when it comes to protecting children from abuse.

“It appears that the church is now advising law enforcement of abuse reports more promptly,” the report states. “Internal review processes have been established. Victims are no longer quite so invisible.”

“But,” the report continues, “the full picture is not yet clear.”

The report also takes issue with the failure to hold bishops and other church leaders accountable for mishandling allegations of abuse, stating, “despite some institutional reform, individual leaders of the church have largely escaped public accountability.”

The head of the U.S. Conference of Catholic Bishops, Cardinal Daniel DiNardo, also condemned sexual abuse and said that U.S. bishops “are shamed by and sorry for the sins and omissions by Catholic priests and Catholic bishops.”

“Priests were raping little boys and girls, and the men of God who were responsible for them not only did nothing,” the report says, “they hid it all.”

“We pledge to maintain transparency and to provide for the permanent removal of offenders from ministry and to maintain safe environments for everyone,” Cardinal DiNardo said in a statement co-signed with Bishop Timothy L. Doherty of Lafayette, Ind., chairman of the Committee for the Protection of Children and Young People. “All policies and procedures regarding training and background check requirements are made publicly available by dioceses.”

The grand jury recommended four changes to help victims of abuse, including cases of abuse that took place too long ago for prosecution or civil lawsuits. The report recommends eliminating the criminal statute of limitations for sexually abusing children, creating a two-year window for victims of sexual abuse to file lawsuits, clarifying penalties for failing to report abuse and banning non-disclosure agreements regarding cooperation with law enforcement.

Addressing his words to the state’s bishops, Mr. Shapiro said: “Statements are one thing. The proof of their claims will be if they support each of the grand jury’s four recommendations.”

“Stand up today, right now, and announce your support for these commonsense reforms,” he continued. “That is the real test that will determine whether or not things have really changed or if it will just be business as usual.”

While the attorney general was critical of Catholic bishops, he praised Bishop Lawrence Persico of the Diocese of Erie, saying he was the only Pennsylvania bishop who testified in front of the grand jury in person rather than submitting a written statement.

The Pennsylvania report echoes the findings of many earlier church investigations around the country—and in other Pennsylvania dioceses—in its description of widespread sexual abuse by clergy and church officials’ concealment of it.

What distinguished this probe was its extraordinary scope: The grand jury scrutinized abuse allegations in six of Pennsylvania’s eight dioceses that, collectively, minister to more than half the state’s 3.2 million Catholics.

The report details allegations in six dioceses going back 70 years.

“Predators in every diocese weaponized the Catholic faith and used it as a tool of their abuse,” Mr. Shapiro said.

He also said that the church’s failure to protect children was exacerbated by law enforcement and a criminal justice system that sometimes looked the other way after claims were made.

The grand jury’s work might not result in justice for Catholics who say they were molested as children. While the nearly two-year probe has yielded charges against two clergymen—including a priest who has since pleaded guilty, and another who allegedly forced his accuser to say confession after each sex assault—the vast majority of priests already identified as perpetrators are either dead or are likely to avoid arrest because their alleged crimes are too old to prosecute under state law.

The Pennsylvania grand jury, convened by the state attorney general’s office in 2016, heard from dozens of witnesses and reviewed more than a half-million pages of internal documents from the Allentown, Erie, Greensburg, Harrisburg, Pittsburgh and Scranton dioceses.

Some current and former clergy named in the report went to court to prevent its release, arguing it violated their constitutional rights to reputation and due process of law. The state Supreme Court said the public had a right to see it but ruled the names of priests and others who objected to the findings would be blacked out pending a September hearing on their claims.

The identities of those clergy members remain under court seal.

A couple dioceses decided to strip the accused of their anonymity ahead of the report and released the names of clergy members who were accused of sexual misconduct. On Friday, the bishop of Pittsburgh’s diocese said a few priests named in the report are still in ministry because the diocese determined allegations against them were unsubstantiated.

Mr. Shapiro spoke to a group that included some survivors of clergy sexual abuse.

“Unlike the Catholic Church, and some in law enforcement, we hear you, and we believe your truth,” he said.

Material from the Associated Press was used in this report. This story has been updated.