What Started the Montague and Capulet Feud?

According to Humanities360.com, it is never revealed who started the feud between the Capulets and Montagues in "Romeo and Juliet," nor does the audience ever learn what caused it. The play does divulge who ended it; Romeo's and Juliet's love for each other and their deaths ended the long-lasting feud.

None of the Capulets and Montagues alive in the play were even around when the feud started. In the Prologue at the beginning of the play, the audience learns that two well-respected families have carried an "ancient grudge." It goes on to say that the play is about Romeo and Juliet and "The fearful passage of their death-mark'd love,/ And the continuance of their parents' rage,/ Which, but their children's end, nought could remove...."