1283: On the second day of Easter which coincided with the day of Passover, a group of Christian men visited revenge upon the jews responsible, of Mainz (Germany), killing ten and pillaging their houses. They were responding to the discovery of the body of a Christian child Archbishop Werner tried to stop them. His intervention kept the jews from suffering even more. The Holy Roman Emperor, Rudolph, conducted an investigation into the affair, confirmed the judgment they had passed on the jews and acquitted the citizens of Mainz of all blame.
This is what one source has to say about it. https://www.jstor.org/stable/1510146?seq=1
This appears to be a main source, unfortunately it's in French.
