PHILADELPHIA — Msgr. William J. Lynn, a former cardinal’s aide, was found guilty Friday of endangering children, becoming the first senior official of the Roman Catholic Church in the United States convicted of covering up sexual abuses by priests under his supervision.

The 12-member jury acquitted Monsignor Lynn, of the Archdiocese of Philadelphia, of conspiracy and a second count of endangerment after a trial that prosecutors and victims rights groups called a turning point in the abuse scandals that have shaken the Catholic Church.

The single guilty verdict was widely seen as a victory for the district attorney’s office, which has been investigating the archdiocese aggressively since 2002, and it was hailed by victim advocates who have argued for years that senior church officials should be held accountable for concealing evidence and transferring predatory priests to unwary parishes.

Monsignor Lynn, 61, sat impassively as the jury foreman announced the verdicts, but relatives behind him were in tears. Judge M. Teresa Sarmina of the Common Pleas Court revoked his bail, and the monsignor stood up, removed his clerical jacket and was led by sheriff’s deputies to a holding cell area. His conviction, on the 13th day of deliberations, could result in a prison term of three-and-a-half to seven years; sentencing is set for Aug. 13.