It was her statement, made after the execution of her mother, that implicated the royal mistress Madame de Montespan in the process, causing Louis XIV to eventually disrupt the whole investigation and classify it as secret.
[1] About the same time, Adam Lesage made his confession, which added that child sacrifice had taken place during the black masses arranged by La Voisin.
The statement of Marguerite Monvoisin was considered vital, in particular as she was not accused to have taken part in any crime personally, but was exclusively a witness.
[1] The remaining members of the organisation were never put on trial, but incarcerated for life by lettre de cachet, and their confessions were sealed.
[1] Étienne Guibourg, Louis Galet, Adam Lesage and Romani were incarcerated at Château de Besançon, and Betrand at Château de Salces; Marguerite Monvoisin, together with her mother's female associates La Pelletière, La Poulain, Magdelaine Chapelain, Marguerite Delaporte and Catherine Leroy, were imprisoned at Belle-Île-en-Mer.