(CNN) -- Monsignor William Lynn, the highest-ranking cleric accused of imperiling children by helping cover up sexual abuse, was found guilty Friday of one count of child endangerment.

He was found not guilty on a second count of endangerment and a conspiracy charge to protect a priest accused of abuse.

The jury was unable to bring a verdict against his co-defendant, the Rev. James Brennan, who was charged with attempted rape of a 14-year-old altar boy and endangering the welfare of a child.

The trial marked the first time U.S. prosecutors charged not just priests who allegedly committed abuses but church leaders for failing to stop them.

"This is a historic day for survivors with Monsignor Lynn convicted of child endangerment," said Marci Hamilton, a Cardozo Law School professor who has written on sexual abuse cases. "Thanks to the brave survivors who came forward."

Removed from the active ministry in 2006, Brennan admitted two years later that he had allowed the youngster to view pornography and sleep in the same bed with him in 1996, according to church investigators' testimony.

Defrocked priest Edward Avery had been due to go on trial with Brennan and Lynn, but he pleaded guilty in March after admitting he sexually assaulted a 10-year-old altar boy during the 1998-99 school year.

The 69-year-old was sentenced to 2½ to five years in prison.

On Wednesday, the jury reported that it was unable to reach a verdict on four of five charges in the high-profile case. Common Pleas Court Judge M. Teresa Sarmina ordered jurors to keep deliberating, according to a source with direct knowledge of the case.

Lynn's conspiracy count relates to an allegation that he schemed with Avery and other archdiocese officials to endanger children.

More than 60 witnesses and alleged clergy abuse victims testified during Lynn and Brennan's criminal trial, which began March 26 and wrapped up May 31, with jury deliberations beginning the next day.

Lynn's defense team argued that their client repeatedly told higher-ups about the alleged abuse and, under strict orders from late Cardinal Anthony Bevilacqua, had no authority to remove priests from the ministry.

Assistant District Attorney Patrick Blessington characterized Lynn's behavior as "disgraceful," "shameful" and "ridiculous," sarcastically calling him a "hero" who put young people in harm's way.

"He actually looked you in the eye and said he put victims first. How dare he?" the prosecutor asked jurors during his more than 2½-hour closing argument.

CNN's Ross Levitt contributed to this report.