Updated at 9:37 p.m. Tuesday, Dec. 17

MINNEAPOLIS (AP) — St. Paul-Minneapolis Archbishop John Nienstedt announced Tuesday that he’s stepping down from public ministry while police investigate an allegation that he improperly touched a boy during a public photo session four years ago, an accusation he strongly denies.

The announcement is the latest blow to an archdiocese that has faced intense scrutiny since a former employee went public with claims that church leaders mishandled sexual abuse allegations.

In a letter posted on the archdiocese’s website, Nienstedt said he is accused of touching the boy on the buttocks during a photo session following a confirmation ceremony in 2009. Confirmation is a Roman Catholic sacrament in which a person becomes a full adult in the eyes of the church.

Nienstedt said he learned of the accusation over the weekend and doesn’t know who is making the claim.

“I presume he is sincere in believing what he claims, but I must say that this allegation is absolutely and entirely false,” Nienstedt wrote. “I have never once engaged in any inappropriate contact with a minor.”

Nienstedt had already been facing public calls for his resignation. In recent months, police have launched investigations into several abuse claims and Nienstedt’s top deputy stepped down.

On Sunday, Nienstedt went before two services at a suburban Minneapolis church to apologize for the mess, telling parishioners: “I am here to apologize for the indignation that you justifiably feel. You deserve better.”

Nienstedt was a polarizing figure in Minnesota even before the clergy abuse scandal flared. Some of the archdiocese’s 825,000 Catholics were angered in 2010 after he mailed out a DVD against gay marriage, and angered again in 2012 after the church spent $650,000 in a failed effort to ban same-sex marriage in Minnesota.

Nienstedt referenced critics in his letter Tuesday, saying he has “taken strong stands on the moral teachings of the Church and been criticized for it.” He added: “I am a sinner, but my sins do not include any kind of abuse of minors.”

Nienstedt said he normally stands for photos with one hand on his staff and the other hand either on the right shoulder of the newly confirmed person, or on a stole that hangs from his chest.

“I do that deliberately, and there are hundreds of photographs to verify that fact,” he wrote.

Accusations of child sex abuse against individual bishops are rare.

Since 2002, when the abuse crisis erupted in the U.S., a few U.S. bishops have stepped down after being accused of molesting minors, including two in the Diocese of Palm Beach, Fla.

Another, Bishop Thomas Dupre in Springfield, Mass., was indicted in 2004 on a child rape charge, becoming the first bishop in the nation to be indicted on an abuse claim. However, the case was dropped after the prosecutor concluded the statute of limitations had expired.

There have also been high-profile cases of bishops being falsely accused, including Cardinal Joseph Bernardin of Chicago in the 1990s.

Bradford Colbert, a resident adjunct professor at William Mitchell College of Law, said such allegations are difficult.

“You shouldn’t be able to convict someone just because someone says it happened, but it becomes a matter of who do you believe?” Colbert said. “It can be really difficult on both sides. … I think the thing to do is just to wait and see and not assume that he’s guilty.”

The archdiocese said in a statement that it learned of the allegation last week after someone within the church brought it forward. The archdiocese said it instructed that person to go to police. St. Paul police said they opened an investigation Monday.

At a news conference Tuesday, St. Paul police Chief Thomas Smith did not provide details about Nienstedt’s case. But he said the archdiocese has not cooperated fully with its other ongoing investigations and has declined to make clergy available to investigators. The archdiocese released a statement saying, “As we have stated repeatedly, the archdiocese seeks to cooperate with the police and all civil authorities.”

The archdiocese said it has asked for an opportunity to meet with St. Paul police, and that the meeting is set for Wednesday.

Auxiliary Bishop Lee Piche will cover Nienstedt’s public duties during the investigation.

Nienstedt’s decision to remove himself from ministry comes as parishes are preparing for Christmas services, when they record their largest attendance of the year and many Catholics make large donations.

“Any time you have an accusation of a serious nature such as this, it can be demoralizing, but especially in the context of all that’s happened in the archdiocese there and around the country,” said Mark Brumley, chief executive of Ignatius Press, which Pope Benedict XVI chose as his English-language publisher.

Cathy Mroszak, a lifelong Catholic in the St. Paul area, said her first thought on hearing the claim against Nienstedt was shock.

“But then I have to be reminded that you have to hear both sides of the story before I make any kind of a judgment,” she said. Mroszak said she knows how confirmation photo sessions work, and how kids are pushed together and rushed through the process.

“I just don’t know how that could be substantiated,” she said of the allegation. “To me, it almost sounds like it’s some of his critics.”