premise: The Whitechapel murders were committed in or near the impoverished Whitechapel district in the East End of London between 3 April 1888 and 13 February 1891. At various points some or all of these eleven unsolved murders of women have been ascribed to the notorious unidentified serial killer known as Jack the Ripper.
hypothesis: The last murder committed by Jack the Ripper occurred on February 13th, 1891.
reason: The article states that the identity of Jack the Ripper is unknown, so it is not possible to say with certainty that no murders were committed by the Ripper after 1891.