Matt Rourke, File, Associated Press
In this Jan. 6, 2014, file photo, Monsignor William Lynn walks from the criminal justice center after a bail hearing in Philadelphia. The Pennsylvania Supreme Court ruling Monday, April 27, 2015, upholds the 2012 conviction of Lynn over abuse committed by a diocesan priest years earlier.

PHILADELPHIA — The state's highest court on Monday reinstated the landmark child-endangerment conviction of a Roman Catholic monsignor who was the first U.S. church official ever prosecuted over his handling of sex abuse complaints.

The Pennsylvania Supreme Court upheld the 2012 felony conviction of Monsignor William Lynn for endangering an altar boy abused by a priest who had been transferred to his parish despite earlier complaints.

Lynn's lawyers have long argued that Lynn, as secretary for clergy, was not responsible for the welfare of the victim under existing Pennsylvania law. However, the state Supreme Court in a 4-1 ruling disagreed, potentially sending the 64-year-old Lynn back to prison.

"(Lynn) was a person supervising the welfare of many children because, as a high-ranking official in the archdiocese of Philadelphia, he was specifically responsible for protecting children from sexually abusive priests," Justice Max Baer wrote.

Lynn had served half of a three-to-six-year sentence when a Superior Court opinion threw out his conviction and led to his release. He has remained on house arrest, in a northeast Philadelphia rectory, while prosecutors appealed.

Defense lawyer Thomas Bergstrom said Lynn may appeal the novel legal issues in the case to the U.S. Supreme Court.

"They're equating Lynn with being in charge of all of the children of the archdiocese, whether they were abused or not," Bergstrom said. "It's troubling to me that this broad approach would be approved by the court."

"He, Lynn, never even knew about this abuse until 2009 and never even knew the child existed, let alone that he was being abused by (the Rev. Edward) Avery," Bergstrom said.

Avery pleaded guilty to abusing the boy but later recanted his apparent admission during Lynn's trial. He was— sentenced to 2 1/2 to five years in prison.

Avery was one of several priests reassigned to unsuspecting parishes despite complaints that Lynn reviewed at the archdiocese, where he held the high-ranking post under two cardinals from 1992 to 2004.

"I did not intend any harm to come to (the boy). The fact is, my best was not good enough to stop that harm," Lynn said at his July 2012 sentencing. "I am a parish priest. I should have stayed (one)."

The same boy also accused another priest and a teacher at the same parish of abusing him. Both were convicted.

Philadelphia Common Pleas Judge M. Teresa Sarmina told Lynn that he should have stood up to his bishops. Instead, she said, he enabled "monsters in clerical garb . to destroy the souls of children."

Lynn has 14 days to decide whether to appeal Monday's ruling.

"I don't think it's fair to assume anybody's going to pick him up tonight," Bergstrom said.

The Philadelphia district attorney's office, which prosecuted him, was expected to comment later Monday.