During the period of the Ripper's crimes, multiple letters were sent to Scotland Yard and various newspapers by individuals purporting to be Jack the Ripper. Many of these "Ripper letters" were preserved, enabling DNA analysis of the stamps and envelopes used once that technology was developed. Author Patricia Cornwell had some of the Ripper letters analyzed by forensic scientists, who determined that mitochondrial DNA which could have belonged to Walter Sickert was present on the letters. A comparison between the Ripper letters and Sickert's own letters also revealed that both contained the watermark of the Aberdeen paper manufacturer Alexander Pirie and Sons. Two findings from some of the last remaining physical evidence in the Ripper case thus point to the same individual: Walter Sickert.