Because of the recent mass killing Milwaukee, I was asked to compile all these incidents in Wisconsin. Having done that, I might as well post it.
Known mass killings (not just mass shootings) in Wisconsin
17 July 1881 Eagle Creek WI, 9 killed (shot) (https://www.newspapers.com/newspage/79786752/)
15 September 1914 Iowa County WI, 7 killed (multiple) (http://murderpedia.org/male.C/c/carlton-julian.htm)
24 November 1917 Milwaukee WI (bomb) 10 killed (bomb) (https://en.wikipedia.org/wiki/Milwaukee_Police_Department_bombing) unsolved
16 January 1942 Plum City WI, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
14 June 1999 Brown County WI, 4 killed (multiple) (http://murderpedia.org/male.G/g/gilbert-michael.htm)
21 November 2004, Meteor WI, 6 killed (shot) (https://en.wikipedia.org/wiki/Chai_Vang)
12 March 2005, Brookfield WI, 7 killed (shot) (https://en.wikipedia.org/wiki/Terry_Ratzmann)
9 June 2007, Delavan WI, 6 killed (shot)
7 October 2007, Crandon WI, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
3 December 2009, Madison WI, 5 killed (shot)
5 August 2012, Oak Creek WI, 7 killed (shot)
21 October 2012 Brookfield WI, 4 killed (shot) (http://murderpedia.org/male.H/h/haughton-radcliffe.htm)
4 May 2015, Menasha, WI 4 killed (shot)
22 March 2017 Weston WI 4 killed (shot) (http://www.nydailynews.com/news/nat....icle-1.3006291
27 May 2017 Chippewa County WI 4 killed (vehicular homicide) (https://wqow.com/news/top-stories/2019/04/05/just-in-jail-time-ordered-for-woman-who-killed-4-in-chippewa-co-crash/)
4 November 2018 Lake Hallie WI, 4 killed (vehicular homicide) (http://www.startribune.com/3-girl-scouts-among-4-killed-in-hit-and-run-crash-near-eau-claire-wisconsin/499507691/)
27 July 2019 Layatte & Lake Hallie WI, 5* killed (shot) (https://www.grandforksherald.com/news/crime-and-courts/4045713-Authorities-search-for-answers-after-Wisconsin-man-fatally-shoots-self-4-others)
27 December 2019 Milwaukee WI 6* killed (vehicular homicide/DWI) (https://www.channel3000.com/report-driver-in-milwaukee-crash-that-killed-4-was-on-meth/)
26 February 2020 Milwaukee WI, 6* killed (shot) (https://www.wisn.com/article/active-shooter-at-millercoors-in-milwaukee/31119531)
there doesn't seem to be anything here