The 77-year-old cardinal, the sixth archbishop of Washington, submitted his resignation, as is mandatory, to the pope when he turned 75 two years ago, but it has not been accepted.
The letter, an extraordinary j'accuse from a one-time Holy See diplomat, also accuses Pope Francis of having initially rehabilitated McCarrick despite being informed of his penchant for young seminarians in 2013, soon after he was elected pope.
The Diocese of Buffalo faces criticism that it has mishandled allegations of sexual misconduct by priests and as it weathers calls for an independent investigation into its practices.