On September 13th, 1440, Jean de Malestroit wrote this -
"...we listened repeatedly to the shocking complaints made by as many good and discreet synodic witnesses...as by several other credible people of probity, at the same time as by many parishioners...as well by the oft-repeated public rumor as as by the preceding denunciations, we discovered that the said nobleman, Milord Gilles de Rais, baron of the said lands in our diocese, had killed, cut the throats of, and massacred many innocent children in an inhuman fashion, and with them committed, against nature, the abominable and execrable sin of sodomy, in various fashions and with unheard-of perversions that cannot presently be expounded upon by reason of their horror, but that will be disclosed in Latin at the appropriate time and place; that he had often and repeatedly practised the dreadful invocation of demons and took care that it be practised; that he sacrificed and made offerings to these same demons, contracted with them; and wickedly perpetrated other crimes and offenses, professing doctrinal heresy against Divine Majesty, in the subversion and distortion of our faith, offering a pernicious example unto many."
(The Trial of Gilles de Rais, by Georges Bataille, translated by Richard Robinson, Amok Books)
On October 13th, the Bill of Indictment was read and, as promised, did indeed go into more detail.
The interrogations of François Prelati, Eustache Blanchet, Etienne Corillaut (Poitou), and Henriet Griart took place between the 16th and the 17th of October. At the point when Jean de Malestroit wrote his initial letter, and when the Bill of Indictment was drafted, the only evidence before him was from the "public rumour" that children had gone missing and from his own interviews with parents and other concerned parties. All that they could tell him was that children were allegedly disappearing in the area, not what had happened to them. What de Malestroit writes is inside information and we are not told where it comes from.
Most writers do not address this discrepancy; I suspect that many do not even notice it. Among the ones who do is Frances Winwar, in The Saint and the Devil, who makes the logical assumption that somebody from Gilles's household had informed on him.
It is possible that François Prelati, Eustache Blanchet or André Buchet (who is named in the Bill of Indictment and was working for Jean V when he allegedly procured a child to be murdered) voluntarily gave information because they realised that Gilles de Rais was about to be arrested and sought to save their own skins. It is equally possible that they were tortured or threatened with torture. Or bribed. Or told that they would save their lives if they made a private confession inculpating their master. It is just as likely that the accusations were pure invention.
The only thing we know for certain is that Jean de Malestroit made allegations, later confirmed by confessions under torture, which he seems to have produced out of thin air and chose not to source.