Thomas of Monmouth’s only known work is the Vita et Passione Sancti Willelmi Martyris Norwicensis, as it is described in the only extant manuscript, Cambridge University Library MS. Add.
[1] In this book, Thomas of Monmouth claims that a boy, William, who has been found dead in the forest, was in fact murdered by Jews in a re-enactment of the crucifixion, and that he therefore is a saint.
In a seminal article, Gavin Langmuir argued that Thomas of Monmouth himself had made the whole thing up.
[2] Subsequent historians have suggested instead that Thomas heard and elaborated on rumors that were already circulating[3] or that the accusation was created as a fake cover story for the murder by a knight called Simon of Novers of a Jew called Eleazar, to whom Simon was in debt.
Rose's conclusions have been challenged by Eamon Duffy, Royal Books and Holy Bones: Essays in Medieval Christianity.