From Hell letter
The reason this letter stands out more than any other is that it was delivered with a small box containing a human kidney, preserved in alcohol. One of Catherine Eddowes' kidneys had been removed by the killer. Medical opinion at the time was split on whether the kidney was likely to have been the same as the one taken from Eddowes. Some officials thought the organ could have been acquired by medical students and sent with the letter as part of a hoax.