(CNN) - It all comes down to one letter, and whether or not Pope Francis read it.
The Pope said as recently as last month he has seen no evidence a Chilean cleric whom he later made a bishop had witnessed another priest abusing teenagers.
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But Francis received a letter detailing exactly that in 2015, according to the author of the letter and a former member of the Vatican's Commission for the Protection of Minors. It was hand-delivered to him, the two people say, by one of his most trusted advisers, Cardinal Sean O'Malley of Boston.
A spokesman for O'Malley referred all questions to the Vatican, which declined to comment.
The Pope's receipt of the letter was first reported by The Associated Press on Monday. The letter's author, Juan Carlos Cruz, has provided a copy to CNN. In it, he details years of abuse, witnessed, Cruz says, by the man Pope Francis later made a bishop, Juan Barros.
"The Pope goes around the world apologizing for the sexual abuse of children," Cruz told CNN, "but he doesn't do a thing."
This is not the first time the Pope has been accused of having a "blind spot" on sexual abuse.
The commission charged with protecting young people, unveiled in 2015 with great fanfare, has languished since December when its term expired. The Vatican says new members are being vetted.
But some of the commission's top recommendations, including a tribunal for bishops accused of covering up sex crimes, have been nixed by Vatican officials, said Marie Collins, a former member of the commission and an abuse survivor. Collins quit the commission last year in frustration over the Vatican's inaction.
Francis also appointed an Australian cardinal to a powerful Vatican post despite concerns from victims' groups. Cardinal George Pell is now on trial on sexual assault charges in his native Australia.
"The status quo is not working," the Rev. Thomas Reese, a Jesuit priest and expert on the Vatican, wrote in a recent column for Religion News Service.
"Pope Francis needs to make dramatic changes in the way in which the Vatican investigates crimes, especially those by bishops."
'A witness to all of this'
"Holy Father, I decided to write you this letter because I'm tired of fighting, crying and suffering."
That's how Cruz, who now lives in Philadelphia, opened his eight-page letter to the Pope.
In the letter, Cruz details years of abuse at the hands of the Rev. Fernando Karadima, a charismatic and powerful figure in the Chilean church. The Vatican removed Karadima from active ministry in 2011 after finding the priest guilty of child sex abuse.
Cruz also writes in his letter to the Pope that Barros "was a witness to all of this I have told you."
Cruz said O'Malley told him he had handed the letter to Pope Francis in 2015. O'Malley also told the Commission for the Protection of Minors, which he led, that he had personally delivered Cruz's letter to the Pope two years ago, said Collins, the former commission member.
Barros has not been accused of sexual abuse and has denied knowing about Karadima's crimes. A representative for Barros has not replied to requests for comment.
In a statement dated January 30, before the AP's report about the letter to the Pope, Barros' diocese issued a statement saying, "The Bishop is in out of town activities and expressed that he is taking everything related to the Pope with faith and joy, asking God for the truth to shine through and especially invoking the Virgin Mary so everyone can reach peace."
Collins said it was a mistake for Pope Francis to appoint Barros a bishop in 2015.
"If he saw what was happening and didn't seem to recognize it as abuse, then putting him in charge of a diocese where he oversees the safety of children is a problem."
In a trip to Chile last month, Pope Francis angered some survivors' groups and Chilean Catholics by forcefully defending Barros, telling reporters that the accusations against the bishop are "slander."
The Pope's statement made international headlines, with supporters of victims of clerical sex abuse challenging his suggestion that "proof" is a prerequisite for believing a victim's account.
Francis' comments elicited a rare public rebuke from O'Malley, the Pope's top adviser on sexual abuse, who called them "a source of great pain for survivors of sexual abuse by clergy."
'He risks it all crashing down'
Questions about Barros dominated a news conference held on the papal plane during the Pope's return to Rome following his visit to Chile and Peru. In response to questions, Francis admitted he had made an error in choosing his words and apologized to victims of clerical sex abuse.
"I apologize for hurting them without realizing it. But I did not intend this," Francis said.
"The word 'proof' was not the best way to approach a pained heart. I would say 'evidence.' In Barros' case, it was studied. It was restudied. And there is no evidence. And that is what I wanted to say. I don't have evidence to convict.
"If I convicted without evidence or without moral certainty, I would commit a crime of being a bad judge," Francis said.
He also reaffirmed his support for Barros -- and his assertion that allegations of sex abuse without evidence are slander.
"I am also convinced he is innocent," he said.
The Pope defended his zero-tolerance position on abuse by clergy, saying that he has received two dozen requests for pardon but has not granted any.
Later, though, Francis seemed to indicate that the church does not have all the evidence in Barros' case. He has dispatched one of the Vatican's top experts on sexual abuse to Chile to investigate.
He will go to Santiago "to listen to those who have expressed the will to submit elements in their possession," the Vatican said in a statement.
Charles Camosy, a professor of ethics at Fordham University, a Catholic school in New York, said it is time for the Pope's defenders to hold him to account.
"Whatever good he's done (and I believe it is a lot)," Camosy tweeted Tuesday, "he risks it all crashing down if he doesn't get this right."
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