Archbishop Carlo Maria Vigano, a former Vatican envoy to the United States, claims he had told Francis of the claims against US cardinal Theodore McCarrick in 2013.
"Corruption has reached the very top of the Church's hierarchy," Vigano said in an eleven-page letter published on Saturday in the National Catholic Register and several conservative US Catholic publications.
But the pope refused to address the allegation on Sunday.
"I will not say a word about that. I think that the communique speaks for itself," Francis said on his plane as he flew back from Dublin to Rome.
Francis told journalists to "read the communique attentively and make your own judgement," referring to Vigano's letter.
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