Police officers, involved in investigations into the murders at the time, later gave their opinions on the number of victims:
Superintendent Thomas Arnold, head of Metropolitan Police H Division (Whitechapel) during the time of the murders, was of the opinion that four victims were murdered by Jack the Ripper, stating in Eastern Post & City Chronicle of 3 February 1892:

   I still hold to the opinion that no more than four of these murders were committed by the same hand. They were the murders of Annie Chapman in
   Hanbury Street, Mrs Nichols in Buck`s Row, Elizabeth Stride in Berner Street and Mary Kelly murdered in Mitre Square
(Arnold`s reference to Mary Kelly is to the alias of Catherine Eddowes and not to Mary Jane Kelly murdered in Miller`s Court).

Inspector Frederick Abberline, in charge of the detectives of ground during murder investigations, implied six murders, stating in
Pall Mall Gazette of 24 March 1903:

   George-yard, Whitechapel-road, where the first murder [of Martha Tabram] was committed
and, earlier, stated in Cassell`s Saturday Journal of 28 May 1892:

   Miller`s Court [where Mary Jane Kelly was murdered] was the last of the real series, the others were imitations

Henry Cox, City of London Police Constable 1881-96, Inspector 1896-1906, retired 1906, also thought there were six murders, stating in Thomson`s Weekly News 1 December 1906:
   The first crime [the murder of Martha Tabaram] took place on August 6; the second [the murder of Mary Ann Nichols] on the last day of the month.
   The third [murder of Annie Chapman] occurred in the beginning of the following month, this time two days later, and the fourth [murder of Elizabeth
   Stride] and the fifth [murder of Catherine Eddowes] were once again on the last day of the month.  The final murder [of Mary Jane Kelly] was again
   on the opening day of the month
(note Mary Jane Kelly was not murdered on the "opening day" but on 9th November 1888).

Edmund Reid, Local Inspector in charge of H Division (Whitechapel) CID during the murder investigations, was of the opinion that Jack the Ripper murdered nine victims, stating in Morning Advertiser of 23 April 1910:
   there were nine murders said to have been committed by `Jack the Ripper`
and expanded on this in Lloyd`s Weekly News of 4 February 1912:

   I still hold to the opinion that no more than four of these murders were committed by the same hand. They were the murders of Annie Chapman in
   the so-called `Whitechapel murders` were not peculiar to that division,  for one was in the City of London [Mitre Square - Catherine Eddowes]; one
   in Bethnal Green [Buck`s Row - Mary Ann Nichols];  four in Spitalfields [George Yard - Martha Tabram; Hanbury Street - Annie Chapman; Miller`s
   Court - Mary Jane Kelly; Swallow Gardens - Frances Coles]; two in St George`s [Berner Street - Elizabeth Stride; Pinchin Street - torso] and only
   one in Whitechapel [Castle Alley - Alice McKenzie

Donald Swanson, Chief Inspector, in overall charge of investigations between 1 September and 6 October 1888, named eleven victims in private notes written about 1891 (cited by Paul Begg and John Bennet, in Jack the Ripper: The Forgotten Victims, published by Yale University Press in 2013): nine "Whitechapel murders" - Emma E Smith; Martha Tabram; Mary Ann Nichols; Annie Chapman; Elizabeth Stride; Mary Janet [sic - Jane] Kelly; Alice McKenzie; Frances Coles; one "alleged attempted murder
- Annie Farmer; one "alleged murder" - Rose Mylett, alias Catherine Millet, alias Davis.