Cornwell has been involved in a continuing, self-financed quest for evidence to support her theory that Victorian painter Walter Sickert was Jack the Ripper. In pursuit of this hypothesis, she has written two books: Portrait of a Killer: Jack the Ripper—Case Closed, published in 2002, and Ripper: The Secret Life Of Walter Sickert, published in 2017. In total, she is said to have spent a reported $6m on Ripper-related research.^[7]

She wrote Portrait of a Killer: Jack the Ripper—Case Closed to much controversy, especially within the British art world and among Ripperologists.^[13][14][15] Cornwell denied being obsessed with Jack the Ripper in full-page ads in two British newspapers and has said the case was "far from closed".^[16][17] In 2001, Cornwell was criticized for allegedly destroying one of Sickert's paintings in pursuit of the Ripper's identity.^[18] She believed the well-known painter to be responsible for the string of murders and had purchased over 30 of his paintings and argued that they closely resembled the Ripper crime scenes.^[18] Cornwell also claimed a breakthrough: a letter written by someone purporting to be the killer had the same watermark as some of Sickert's writing paper.^[18] Ripper experts noted, however, that there were hundreds of letters from different authors falsely claiming to be the killer, and the watermark in question was on a brand of stationery that was widely available.^[14]

French art expert Johann Naldi validates the author's theory, claiming to have found a portrait that he attributes to the French painter Jacques-Émile Blanche.^[19][20][21] For Naldi, the discovery of this painting, which depicts a man who appears to share Sickert's features, is "visual confirmation of Patricia Cornwell's theory".^[22]