A special panel from the Archdiocesan Review Board investigated the allegations and concluded there was sufficient evidence to suggest that the abuse had occurred, he said.
"Today is time of pain and sorrow," DiNardo said. "Sorrow obviously for the person who made the allegation and sadness for the parishes and for the priest involved too."
Horn, 63, has been on leave from his current post as pastor of St. Luke the Evangelist Church since the allegations were made, DiNardo said. He had served as pastor of that church since 1994.
DiNardo said Horn was receiving care at a "facility that helps with some treatment and monitoring" for priests accused of abuse. He declined to say whether Horn has admitted or denied guilt.
Horn was ordained in 1973 and was highly respected within the archdiocese, DiNardo said.
DiNardo said the archdiocese will refer Horn's case to church officials in Rome, and he could face a canonical trial. The cardinal said the case also has been referred to the district attorney's office but it is likely that too much time has passed for him to be prosecuted.