http://www.aish.com/jl/h/cc/48950796.html
...Here is one eyewitness account of an attack on the Jewry of Mainz in May of 1096. This comes from The First Crusade by August Krey, and it is a letter written by a Jew who survived:
“The Jews of the city, knowing of the slaughter of their brethren fled in hope of safety to the Bishop of Ruthard. They put an infinite treasure in his guard and trust having much faith in his protection. He placed the Jews in a very spacious hall in his own house that they might remain safe and sound in a very secure and strong place.”
“But ... the band held council, and after sunrise attacked the Jews in the hall with arrows and lances, breaking down the bolts in the doors. They killed the Jews, about 700 in number who in vain resisted the force of an attack of so many thousands. They killed the women also and with their sword pierced tender children whatever age and sex...”
This is how about 30%-50% of the Jewish community of Europe met its end. Some 10,000 Jews of an estimated population of about 20,000-30,000 were slaughtered by Crusaders mobs.
The perpetrators of this were the mobs that were inspired to join a “people’s” crusade, and they were living off the land as they marched to Constantinople. Their looting and pillaging was not confined to the Jews, although the hated Jews had the most money to steal. It is well to note that when the mobs appeared in Constantinople, the Greeks transported them across the Straits where they were slaughtered by the Turks.