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Hundreds of children were abused in secrecy by Pennsylvania Catholic clerics over 40 years, according to a grand jury reportFlickr/Caroline

Two bishops in Pennsylvania covered up the sexual abuse of hundreds of children by more than 50 Catholic priests over four decades, a grand jury has found.

Abusive priests — described as "wolves disguised as shepherds" — were shuffled among parishes, according to a scathing 147-page grand jury report. One priest was sent to a school for boys after a parent complained about the sexual abuse of her son.

Victims were paid off or even threatened with excommunication from the church if they signalled that they intended to complain to police.

The church need not have worried about action taken by police and civil authorities because they "would often defer to the diocese" in such cases, the report said.

One local police chief testified that diocesan officials wielded significant political power, and he believes a Monsignor was in part responsible for his appointment as head of the police department.

Not one of the crimes against the children can currently be prosecuted because the abusers have either died or the statue of limitations has run out. One of the grand jury's recommendations is that the state legislature extend the time allowing a criminal complaint to be lodged to allow adults to seek prosecution of sex abuse they suffered as a child.

Victims' "souls were killed as children," Pennsylvania Attorney General Kathleen Kane said in a statement. "They weren't out playing baseball; they were trying to avoid priests.

"These predators desecrated a sacred trust and preyed upon their victims in the very places where they should have felt most safe," she added. "Just as troubling is the cover-up perpetrated by clergy leaders that allowed this abuse to continue for decades."

Bishops Joseph Adamec and James Hogan of the Altoona-Johnstown Diocese were accused by the grand jury of covering up an astonishing level of child sexual abuse. Hogan led the diocese from 1966 to 1986, and died in 2005. His successor Adamec retired in 2011. He refused to testify before the grand jury.

The investigation was based on a trove of secret diocese documents discovered after a search warrant was issued, the Christian Science Monitor reported.

Among the papers found was a chilling "cheat sheet" list of payments for child sex abuse. "Above clothing, genital fondling," for example, netted victim payments starting at $10,000 (£7,000), while victims of forced oral sex were paid up to $50,000 (£35,000), and "sodomy, intercourse" victims were paid $50,000 to $175,000 (£124,000) according to the diocesan records.

Adamec has denied any culpability in the crimes. His attorney said Adamec "deeply regrets any harm that has come to children who were victimized," but insists the claims against the former bishop are unfounded.

"This is a painful and difficult time," said Bishop Mark Bartchak, who currently runs the diocese, which is home to 90,000 Catholics. "I urge the faithful to join me in praying for all victims of abuse."

Bartchak has not been accused of any wrongdoing and has worked to suspend some of the priests named in the grand jury report.

The Catholic church is reeling from a massive scandal of sex abuse of children by clergy.

The Oscar-winning movie Spotlight examined the Boston Globe investigation launched in 2002 that eventually uncovered the abuse hundreds of children in Massachusetts.