Webster's principal source was in William Painter's The Palace of Pleasure (1567), which was a translation of François de Belleforest's French adaptation of Matteo Bandello's Novelle (1554). Bandello had known Antonio Beccadelli di Bologna in Milan before his assassination. He recounted the story of Antonio's secret marriage to Giovanna after death of her first husband, stating that it brought down the wrath of her two brothers, one of whom, Luigi d'Aragona, was a powerful cardinal under Pope Julius II. Bandello says that the brothers arranged the kidnapping of the Duchess, her maid, and two of her three children by Antonio, all of whom were then murdered. Antonio, unaware of their fate, escaped to Milan with his oldest son, where he was later assassinated by a gang led by one Daniele Bozzolo.
Webster's play follows this story fairly faithfully, but departs from the source material by depicting Bozzolo as a conflicted figure who repents, kills Antonio by mistake, then turns on the brothers killing them both. In fact the brothers were never accused of the crime in their lifetimes and died of natural causes.