Sipe estimated that 50 percent of priests are living unchastely, and 6 percent of clergy are abusers, though those estimates have faced frequent challenges from other researchers, including a 2004 study by the John Jay College of Criminal Justice, commissioned by the U.S. bishops' conference.
Sipe wrote to Bishop McElroy in 2016, listing allegations against half a dozen bishops – including then-Cardinal Theodore McCarrick – and warning of a broader problem of chastity violations among clergy.
"Sooner or later it will become broadly obvious that there is a systemic connection between the sexual activity by, among and between clerics in positions of authority and control, and the abuse of children," Sipe wrote in the letter.
"When men in authority – cardinals, bishops, rectors, abbots, confessors, professors –are having or have had an unacknowledged-secret-active-sex life under the guise of celibacy an atmosphere of tolerance of behaviors within the system is made operative."
The letter, which was published on Sipe's website, drew media attention following the psychotherapist's death earlier this month.
On Aug. 17, McElroy issued a public statement on the matter, noting Sipe's death on Aug. 8. He said that Sipe had requested to meet with him about clergy sex abuse in 2016.
Over the course of "two long, substantive, cordial and frank discussions about the history of clergy sexual abuse in the United States," McElroy said, Sipe made allegations against several bishops – including some who were then in ministry – and said that he was planning to approach the apostolic nuncio, Archbishop Christophe Pierre, about the issue.
McElroy said he raised concerns that some of the Sipe information may be inaccurate.
"In two instances we discussed, I had certain knowledge of individuals being investigated and cleared yet he still leveled accusations against them," the bishop said.
"Dr. Sipe stated that he was making many of his allegations against existing bishops based on information that he had received from his work in legal cases on behalf of survivors of abuse," McElroy said, but asked if he could share specific corroborating documents, Sipe said he was unable to do so.
After Sipe requested a third meeting but was told by the McElroy's assistant that the bishop could not meet with him that month, he hired a process server who came to the office, posing as a donor wishing to hand-deliver a check, McElroy said. The process server delivered a letter from Sipe.
McElroy said he did not respond to that letter because Sipe's use of a process server, and apparent dissemination of the letter, made him untrustworthy.
"After I read it, I wrote to Dr. Sipe and told him that his decision to engage a process server who operated under false pretenses, and his decision to copy his letter to me to a wide audience, made further conversations at a level of trust impossible."
Sipe's July 28, 2016 letter warned of a widespread culture of illicit sexual activity among clergy. Pointing to his time as a staff member at three major seminaries, he said that patterns of sexual behavior are often established "during seminary years or in early years after ordination when sexual experimentation is initiated or sustained."
"A serious conflict arises when bishops who have had or are having sexually active lives with men or women defend their behavior with denial, cover up, and public pronouncements against those same behaviors in others," he said. "Their own behavior threatens scandal of exposure when they try to curtail or discipline other clerics about their behavior even when it is criminal as in the case with rape and abuse of minors, rape, or power plays against the vulnerable."
In the letter, Sipe listed allegations against several bishops, including reports of misconduct by Archbishop John Neinstedt and Bishop Robert Brom, abuse by Bishop Thomas Lyons and Bishop Raymond Boland, and cover-up by Cardinal Roger Mahony.
He also said that he had interviewed 12 priests and seminarians who described sexual advances and activity on the part of then-Cardinal Theodore McCarrick.
Sipe referenced a settlement against McCarrick, which he said described the cardinal's sexual behavior and included correspondence from him.
McCarrick's sexual propositions and harassment were covered up by intimidation, Sipe said, with priests and seminarians unwilling to speak up about it, for fear of risking their reputation and facing retaliation.