Christopher Tyerman describes a series of attacks on Jews in the Rhineland en route to the Frist Crusade.
In 1096, some contingents on the First Crusade attacked Jews in the Rhineland. The Jewish communities in the Rhineland had been growing over the previous century and a half. They were prosperous, they were under the protection of the church, they were under the protection of the German Emperor. However, many of the local German crusaders may have been in debt to Jewish bankers; there were obviously social tensions in urban communities along the Rhine. These spill over, under the umbrella of the Crusade, into anti-Jewish violence.
This was not a goal of the Crusade – but if you’re saying, we are fighting the enemies of Christ … the mass says, the Jews crucified Christ. So there is a sort of an easy syllogism, an easy communication, between the actual message of the Crusade and how it’s taken up by people who wish to air a social grievance. It’s got nothing to do with the Crusades at all. And these massacres are quite restricted to the Rhineland, to certain German contingents, to certain French contingents. But they are disapproved of by most of the local authorities in Germany, and by most of the Crusade leaders.