Pope Pius may have known about Nazi gas chambers as early as 1942

The letter is highly significant because of the date of Koenig's letter. The correspondence arrived in Pius' office in the days after the ghetto in Rava-Ruska was emptied between December 7 and 11, 1942. 

The letter was also discovered by an in-house Vatican archivist and made public with the engagement of Holy See officials.

It can't be certain that Pius saw the letter however Leiber was Pius' top aide and had served the pope when he was the Vatican's ambassador to Germany during the 1920s, suggesting a close working relationship especially concerning matters related to Germany.

Vatican archivist Giovanni Coco told Italian newspaper Corriere the importance of the letter was "enormous, a unique case" because it showed the Vatican had information that labour camps were actually death factories.

He added: “The newness and importance of this document derives from a fact: now we have the certainty that the Catholic Church in Germany sent Pius XII exact and detailed news about the crimes that were being perpetrated against the Jews.”

Asked by the Corriere interviewer if the letter showed that Pius knew, Coco said: "Yes, and not only from then."

The letter, according to Coco, was only recently handed over to the central archives where he works.

Supporters of Pius say he worked behind the scenes to help Jews and did not speak out in order to prevent worsening the situation for Catholics in Nazi-occupied Europe. 

However, his detractors say he lacked the courage to speak out on information he had despite pleas from Allied powers fighting Germany.