Specifically, Viganó says that Pope Benedict XVI imposed private sanctions on Theodore Cardinal McCarrick in 2009 - and that Francis lifted them at the urging of his staff, then proceeded to rely on McCarrick's counsel in naming new USA bishops.
Archbishop Vigano, 77, a conservative whose hardline anti-gay views are well known, urged the reformist Pope to resign over the issue and what he called the "conspiracy of silence" about McCarrick.
The Pope was asked about the accusations by reporters on his flight back to Rome after this weekend's Papal visit to Ireland. He said the letter, with its unsubstantiated claims, "speaks for itself", according to the National Catholic Reporter.
Archbishop Vigano criticized Pope Francis for not taking action against Cardinal McCarrick after he claimed he told the pope of the allegations in 2013.
He said the pope "knew from at least June 23, 2013, that McCarrick was a serial predator", adding that "he knew that he was a corrupt man, he covered for him to the bitter end". "The cardinal, muttering in a barely comprehensible way, admitted that he had perhaps made the mistake of sleeping in the same bed with some seminarians at his beach house, but he said this as if it had no importance", Archbishop Vigano wrote.
Vigano also mentioned in his letter "two documents from the same Littleton, in which he recounted his tragic story of sexual abuse by (McCarrick)".
"In doing so, he put other seminarians at risk", he added.
John McCain, influential USA senator dies aged 81
Bush had been asked to give eulogies, people close to both former presidents and a source close to the senator confirmed to CNN . Meanwhile, Foreign Minister Shah Mahmood Qureshi has expressed his condolences on the passing of USA senator John McCain .
Edward McFadden, spokesperson for archdiocese of Washington, disputed the notion that Wuerl ignored allegations about McCarrick.
He said he told Francis about the accusations against McCarrick in June 2013. Wuerl, however, has also been accused of helping to protect abusive priests when he served as Pittsburgh's bishop between 1988 and 2006. Write to us in the Comments Section or on our Facebook page.
Strickland's letter did not say why he believes the allegations are credible.
A devastating report earlier this month accused more than 300 priests in the U.S. state of Pennsylvania of abusing more than 1,000 children since the 1950s.
The McCarrick case, which also included a charge of child sex abuse, has rocked the Catholic Church in the United States, The American church is also reacting to anger in the pews over a recent Pennsylvania grand jury report that said hundreds of Catholic priests in the state sexually abused more than 1,000 children over a period of 70 years.
The timing of the letter's release - right in the middle of Francis's landmark trip to Ireland - has raised speculation of a campaign against the Argentinian pontiff by conservatives in the Church. Thousands more gathered in the city center for protests against clerical sexual abuse. He answered questions on abortion, sexuality and the child abuse crisis in the Church - but then someone asked him about a letter that accuses Francis himself of covering something up.
Pope Francis has asked forgiveness for the multitude of abuses suffered by victims in Ireland at the hands of the church over decades, as he concluded a tour of the once deeply Catholic country.