Secret documents today placed Pope Benedict XVI at the centre of allegations of cover-up by the Catholic church of a priest sex abuse scandal in the United States.

Letters from the Vatican show that the enforcement department headed by the pontiff, then Cardinal Joseph Ratzinger, took control of the efforts to bring paedophile priest Father Lawrence Murphy to justice, first ordering in 1997 that a church trial could only go ahead in conditions of total secrecy and then changing tack in 1998 and quashing it.

The change of heart came after Fr Murphy, who had sexually abused 200 vulnerable youths at a school for the deaf in Milwaukee between 1950 and 1974, wrote directly to the future Pope begging for mercy.

Monsignor Tarcisio Bertone, then Cardinal Ratzinger's deputy at the Congregation for the Doctrine of the Faith, responded by writing to the US church suggesting they take only lesser, pastoral measures against Fr Murphy.

When the US church refused, insisting on a canonical trial, Monsignor Bertone called a summit meeting in Rome with the US bishops and told them bluntly to call the trial off and merely prevent the Fr Murphy from celebrating Mass.

"This Dicastery has every hope that the priest in question will demonstrate a willingness to cooperate in the solution to this painful case which will favour the good of souls and avoid scandal," wrote Monsignor Bertone.

A further letter from Monsignor Bertone later in 1998, after Fr Murphy had died of natural causes, returns to the same theme of preventing news of sex abuse leaking out to the media.

"This Dicastery commends Fr Murphy to the mercy of God and shares with you the hope that the Church will be spared any undue publicity from this matter," writes Monsignor Bertone, in a letter that is entirely silent on the subject of the suffering of Fr Murphy's victims.

The letters, published today on the website of the New York Times , have added to the mounting accusations that Pope Benedict had, at the least, made no effort to halt the widespread cover up of cases of sex abuse and appears in fact to have encouraged and even led the Catholic church's climate of secrecy.

In 2001 Cardinal Ratzinger issued a letter to every diocese in the Catholic Church, conceding that sex abuse was a grave crime but insisting: "Cases of this kind are subject to the pontifical secret."

The Vatican says that this meant simply that trial judges were obliged not to reveal any details of the case. The letter appears however to have been widely interpreted by dioceses throughout the Catholic world to mean that the Church should avoid publicity at all costs over paedophile priests, to the extent of failing to report offenders to the police. This is no evidence that the future Pope did anything to correct this.

Today a group of American clerical abuse victims were arrested as, flanked by photos of other clerical abuse victims and a poster of Ratzinger, they tried to hold a press conference outside the Vatican to denounce Benedict's handling of the Murphy case.