Archbishop Carlo Maria Viganò, the Vatican's ambassador to the United States at the time, blesses the altar at the start of Mass at the Basilica of the National Shrine of the Immaculate Conception in Washington in 2016. (Jahi Chikwendiu/The Washington Post)

Only months before being recalled by Pope Francis in 2016, Archbishop Carlo Maria Viganò — the Holy See’s former ambassador to the United States — declared in a Baltimore speech that silence was “imprudent.”

In an 11-page letter made public on Sunday, Viganò, a diminutive 77-year-old who is no stranger to church intrigue, followed his own advice — explosively outlining an alleged extraordinary coverup of sexual abuse allegations against the former archbishop of Washington, Theodore McCarrick. It went as far up, Viganò claimed, as the sitting pope.

Pope Francis “knew from at least June 23, 2013 that McCarrick was a serial predator,” Viganò stated. “He knew that he was a corrupt man, he covered for him to the bitter end.”

Viganò also claimed that the pope emeritus, Benedict XVI — a noted conservative — knew of the accusations. Neither the current pontiff nor his predecessor has confirmed or denied those claims.

In an institution that expects total fealty to the throne of St. Peter, observers seeking to make sense of Viganò’s unprecedented reproach see evidence of the internal wars between conservatives and liberals that have defined Francis’s tenure.

“As a person, you could say Viganò is a little rough around the edges,” said the Rev. Tom Reese, author of “Inside the Vatican: The Politics and Organization of the Catholic Church.” He added that “this is like a member of the CIA writing a kiss-and-tell book. This just never happens.”

Especially in recent months, Viganò — who was denied by Francis a traditional elevation to cardinal after his time as U.S. ambassador — has openly aligned himself with the pope’s fiercest critics. Those conservative prelates have decried the first Latin American pontiff’s outreach to gay and divorced Catholics, pointedly questioning his positions.

During his time in Washington, Viganò emerged as a conservative darling who arranged a surprise meeting between Francis and Kim Davis, the Kentucky county clerk who spent six days in jail for refusing to issue marriage licenses to same-sex couples. Yet during his U.S. tenure, Viganò was not above reproach on the issue of sexual abuse. According to documents from a church-sponsored investigation, Viganò tried to cover up a case against the former archbishop of St. Paul, Minn., John Nienstedt, who faced allegations of misconduct with men.

“Archbishop Viganò is certainly an unlikely reformer,” said Melanie Sakoda, a board member of the group SNAP, which advocates for the victims of clergy abuse. She added, “Conservatives within the Church have been attempting to use the current crisis to condemn Pope Francis and other liberal elements within the church. Conservatives jumped on the allegations against Cardinal Theodore McCarrick, a liberal, while ignoring similar allegations against Archbishop Nienstedt, a conservative.”

Born Jan. 16, 1941, in Varese, Italy, Viganò was ordained a priest in March 1968 and quickly found his niche in diplomacy. The soft-spoken but stern cleric worked in Vatican missions in Iraq and England and served as apostolic nuncio — or ambassador — to Nigeria under Pope John Paul II in 1992.

He subsequently rose to major posts inside the Roman curia — the rarefied bureaucracy that runs Vatican City. His letters to Pope Benedict were among the most explosive documents stolen by the pope’s butler and published in the 2012 “Vatican leaks” scandal that offered an insider’s glimpse at the Italian soap opera of infighting and moral failures within the highest reaches of the church.

In his letters, Viganò suggested that he so rattled the Holy See on his crusades to clean up corruption — including allegations of millions of dollars being overspent on contracts — that he was being forced into reassignment. Although typically a coveted position, Viganò saw his transfer as ambassador to Washington as a step down that would hinder reforms.

“My transfer right now,” he wrote in one pleading letter to Benedict, “would provoke much disorientation and discouragement in those who have believed it was possible to clean up so many situations of corruption and abuse of power that have been rooted in the management of so many departments.”

Intrigue followed him to the United States, where Viganò fast became known as the unyielding conservative lion who, apparently unbeknown to Francis, arranged the 2015 meeting with Davis. The Vatican later distanced itself from the meeting, bluntly saying the pope’s encounter with Davis should not be “seen as endorsement.”

Although senior church officials often retire at age 75, many do stay on. Viganò wasn’t one of them. In 2016 — only a few months after the Davis meeting — he was replaced.

Since returning to Italy, Viganò has become a fixture at conservative forums attended and often organized by some of Francis’s top critics — including Cardinal Raymond Burke, who has openly challenged the pope, especially on Francis’s outreach to divorced and gay Catholics. Burke and Viganò are among those in the hierarchy who have denounced the influence of homosexuals in the church, with Vigano in his text denouncing one senior Vatican official for “promoting homosexuals into positions of responsibility.”

During an antiabortion conference in Rome this summer, Viganò sat next to Burke, and took a thinly veiled swipe at the pope’s leadership.

“The thing that I realize being with you,” Viganò told the conservative delegates, “is the great desire to have a strong leadership in the church that can unite all of us.”

In his 11-page testimony, Viganò called for Francis to resign, suggesting the pope had also refused to take his advice on the elevation of bishops in the United States.

“It is hard not to conclude that the real source of his bitterness was that he is upset with the pope,” said Michael Sean Winters, a columnist at the National Catholic Reporter, a leading American Catholic outlet. “He won’t be a cardinal. If you’re nuncio to countries like the U.S. or France, at the end of your tenure, you go back to Rome and you are generally made a cardinal.”

Francis issued a vaguely worded response on Sunday, saying: “I read the statement this morning and, sincerely, I must say this to you and anyone interested: Read that statement attentively and make your own judgment.”

On Monday, at least one senior official backed Viganò. The Rev. Jean François Lantheaume, the former first counselor at the apostolic nunciature in Washington, said Viganò “tells the whole truth. I am a witness.”

“The bishops are neither unscathed nor untouchable,” he said in an exchange posted on his Facebook page. “They are just as sinful as others!!! Let’s say it once and for all.”

Faiola reported from Miami. Stefano Pitrelli in Moena, Italy; Rachelle Krygier in Caracas, Venezuela; and Terrence McCoy in Washington contributed to this report.