Two priests are going on trial in the Vatican’s criminal tribunal this week, one accused of sexually abusing an altar boy who served at papal Masses in St. Peter’s Basilica, and the other accused of covering it up.

The trial, confirmed Monday by the Holy See press office, marks the first known time that the Vatican has criminally prosecuted a case of sexual abuse that allegedly occurred within its walls.

The proceedings starting Thursday were forced on the Holy See after victims and a whistleblower went public in 2017. Their stories undermined Pope Francis’ pledges of “zero tolerance” for abuse because the alleged crimes occurred in his own backyard and had gone unpunished for years.

The case concerns the closed world of the St. Pius X youth seminary, a palazzo inside the Vatican walls just across the street from where Francis lives. The seminary serves as a residence for about a dozen boys, aged 12 to 18, who serve as altar boys at papal Masses.

A onetime seminarian, the Rev. Gabriele Martinelli, is accused of molesting a younger seminarian when he himself was a senior altar boy at St. Pius X. The Rev. Enrico Radice was the rector of the seminary at the time, and is accused of aiding and abetting the crime.

Neither Martinelli nor Radice has responded publicly to the accusations. The order that runs the seminary, the Opera Don Folchi, has said the allegations were “mud” and “calumny,” though the diocese of Como where both men are now priests has removed them from ministry with minors pending an outcome of the case.

The scandal is particularly grave because the allegations of abuse were known since at least 2012 but were covered up for years by the Vatican and the diocese of Como, until they were exposed by Italian journalists Gaetano Pecoraro and Gianluigi Nuzzi in 2017.

They relied on the eyewitness testimony of the victim’s roommate, Kamil Jarzembowski, who was kicked out of the seminary after first reporting the abuse privately to church authorities in 2012.

Jarzembowski has said he saw his roommate repeatedly molested by Martinelli at night when Martinelli would enter their dorm room. He has said the molestation began when both were minors and continued after Martinelli turned 18.

Speaking Monday to The Associated Press, Jarzembowski said he was hopeful justice would finally be served.

“I hope it’s a just trial, in front of impartial judges,” he said. “I want to go into the tribunal and look them in the eye.”

Jarzembowski has been called as a witness for the prosecution.

Usually if it investigates sexual abuse at all, the Catholic Church prosecutes accused priests according to the canon law, the church’s in-house legal system that can impose a maximum penalty of defrocking.

In this case, because the alleged crimes occurred within the confines of Vatican City, the city state’s criminal prosecutors also have jurisdiction and decided to charge the two priests and bring the case to trial.

The first hearing Thursday is expected to be largely technical in nature. The Vatican is allowing a small pool of two reporters to attend, limiting the number because of COVID-19 restrictions.

The diocese of Como, which ordained Martinelli a priest after initially clearing him of the allegations, now insists that it restricted his ministry after “new elements” emerged in 2017. The diocese says it was committed to finding the truth “even recognizing possible delays or omissions committed in the past.”

A series of bishops, including Cardinal Angelo Comastri, the archpriest of St. Peter’s Basilica, said they investigated the allegations, but no one ever interviewed Jarzembowski’s roommate.

The case is emblematic of the inherent conflicts of interest in the canon law system and the church’s refusal to require abuse be reported to impartial police. Bishops are tasked with investigating allegations of abuse, but they also invest time and money for training the accused priests and need them to fill Mass schedules and perform pastoral work.