The earliest document relating to the abbey is a land grant of Louis the Pious dated December 813, now in the Archivio di Stato of Siena. The abbot received full temporal powers in an imperial document of about 952. Following a bequest of Bernardo degli Ardengheschi, construction of the present church was begun before 1118, a date which is inscribed on the altar step and on a column to the left of it. Parts of the earlier structure remain visible in the crypt and in the so-called Cappella Carolingia, or Carolingian chapel.