It's been almost two years since a Philadelphia grand jury probe of Catholic clergy sex abuse of children resulted in charges against four priests and a teacher.

On Monday - after last year's landmark, three-month trial ended in the first criminal conviction of a church administrator for covering up the crimes of deviate priests - the last two defendants, the Rev. Charles Engelhardt and former parochial schoolteacher Bernard Shero, are to go to trial.

Prosecutors, defense lawyers, and Common Pleas Court Judge Ellen Ceisler will begin winnowing a large group of candidates down to a jury of 12 plus several alternates.

Lawyers for the two men wanted them tried separately from the other defendants. One reason was to avoid tarring the two in a trial that focused largely on church records documenting how archdiocesan officials for decades ignored victims to protect the institution.

The first trial judge, M. Teresa Sarmina, also agreed to sever the two because neither was directly supervised by archdiocesan officials. Shero was a lay teacher, and Engelhardt was assigned to Philadelphia under his independent order, the Oblates of St. Francis de Sales.

But a separate trial had its downside: three months of blanket news coverage that arguably left no one in Philadelphia ignorant of the case against the church.

To minimize further pretrial publicity, Ceisler retained the first trial's gag order on lawyers and witnesses and also ordered all motions to be filed under seal and all pretrial hearings to be held in chambers, out of public earshot.

The little that is known is that jury selection for this trial could take longer than the estimated two weeks of testimony.

The first trial ended June 22, when the jury found Msgr. William J. Lynn guilty of child endangerment, making him the first church administrator convicted in connection with a priest's sexual abuse of a child.

Lynn, 62, who as secretary for clergy from 1992 to 2004 was responsible for investigating allegations against priests, was sentenced to three to six years for child endangerment.

He is serving his term in the Waymart state prison in northeastern Pennsylvania, but has filed an appeal.

The case against Engelhardt and Shero involves one of the first trial's most salacious episodes: the serial sexual assault of a 10-year-old altar boy from Northeast Philadelphia in the late 1990s.

Identified in the grand jury report as "Billy Doe," the boy was a fifth-grader at St. Jerome's parish.

According to the grand jury, Billy was first abused by Engelhardt after serving at an early-morning weekday Mass at St. Jerome's.

Engelhardt allegedly caught the boy in the church sacristy drinking leftover sacramental wine. Instead of scolding the boy, the priest allegedly poured him more wine, showed him pornographic magazines, and told him he would soon start sessions to teach him to "become a man."

About a week later, again in the sacristy, Engelhardt allegedly molested Billy and performed oral sex on the boy.

Two weeks later, Engelhardt again approached Billy, who rebuffed him, and Engelhardt then left him alone.

But a few months later, the report says, Billy was accosted by another priest, the Rev. Edward Avery, who was chaplain at nearby Nazareth Hospital and who lived at St. Jerome's rectory.

Avery had a history of sexually abusing children, and in 1992, Lynn ordered him to go to St. John Vianney, the archdiocesan hospital for priests with sexual or drug and alcohol problems. Upon Avery's release, doctors recommended he not serve where he could have access to children. Lynn assigned him to Nazareth, but then-Archbishop Cardinal Anthony J. Bevilacqua said Avery could live at St. Jerome's.

The grand jury report alleges that Avery told Billy he had heard about the boy's "sessions" with Engelhardt. Avery allegedly performed oral sex on the boy and penetrated him with a finger.

After another assault, the report says, Billy avoided serving Mass with Avery.

After summer break, Billy returned to St. Jerome's for sixth grade and was assigned to Shero's classroom.

The grand jury report alleges that one day, Shero offered Billy a ride home from school. Shero allegedly stopped the car and orally and anally raped the boy. He told Billy to get out of the car and walk home.

Billy, now 23, testified at Lynn's trial that the abuse blighted his childhood. Ashamed and too frightened to tell anyone, he was expelled from two high schools, tried to kill himself, and spent years addicted to heroin and pills.

At trial, Lynn testified that he regretted letting Avery live at St. Jerome's because of what happened to Billy.

And Avery - charged along with Lynn, Engelhardt, and Shero after the 2011 report - pleaded guilty on the eve of the first trial to sexually assaulting Billy in 1999.

Avery, now 70, was sentenced to 21/2 to five years and is in a state prison in Western Pennsylvania.

Though Avery pleaded guilty, he could prove the most thorny problem prosecutors now face.

In August, prosecutors said they might call Avery as a witness against Engelhardt and Shero although Avery's guilty-plea agreement did not require him to testify.

In September, in a failed bid to get Lynn released on bail pending appeal, Lynn's lawyers contended that Avery, corroborated by a lie detector, had recanted his guilty plea involving Billy Doe. Avery, Lynn's lawyers said, denied knowing Billy.

Prosecutors have said that they never withheld evidence from Lynn's defense team because they never knew of a polygraph exam or that Avery denied assaulting Billy.

The fifth person charged after the 2011 grand jury report, the Rev. James J. Brennan, 49, is to be retried March 6 in the attempted rape of a 14-year-old boy in 1996. Brennan was tried with Lynn, but the jury was unable to reach a verdict, resulting in a mistrial.

Contact Joseph A. Slobodzian at 215-854-2985, jslobodzian@phillynews.com, or follow @joeslobo on Twitter.