The investigator's report released Thursday found that a man sent a letter to the Rev. Pat Egan in 1990 saying that Egan sexually assaulted him the previous year, when the victim was about 25 years old.
The diocese learned of the accusation in the early 1990s but didn't investigate because the victim wasn't a minor at the time he said he was assaulted, according to the investigation conducted by the law firm Honigman LLP.
"I repeat publicly now what I have said privately and personally to the victim in question: I am deeply sorry for the Diocese's past failure and all should know that the allegation would have been handled differently today," Bishop Earl Boyea said in a statement.
In 2014, Egan was also accused of sexual assault during boxing sessions. Following an investigation, the diocese revoked Egan's priestly facilities and extern status last year, the Lansing State Journal reported.
"Fr. Pat Egan's actions disgraced the holy priesthood and betrayed the trust placed in him," said David Kerr, spokesman for the Diocese of Lansing. "The independent review carried out by Honigman made clear to us that, while our current procedures and policies are appropriate, they could be improved by a more independent and transparent process overseen by a lay-led Adult Review Board."
The Michigan attorney general's office is still investigating sexual abuse of minors by Catholic clergy across the state.
Information from: Lansing State Journal, http://www.lansingstatejournal.com