==1900==
24 July 1900 New Orleans LA 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas)) Not clear if all the killings happened in one emotional event.
==1901==
24 February 1901 Uniontown WA 6 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
21 March 1901 Coldbrook Spring MA, 6 killed (beaten) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1902==
6 April 1902 Tuscumbia AL, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
15 June 1902 Brandywine MS, 6 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1903==
12 March 1903 Bellefontaine MO, 8 killed (assault) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
13 August 1903 Winfield KS, 9 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1905==
30 September 1905 Cambridge IL, 8 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
24 May 1905 San Rafael CA, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
24 November 1905 Littleton IA, 6 killed (beaten) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1906==
17 January 1906 Pembroke NH, 8 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
13 October 1906 Newport TN 8? (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
13 November 1906 Asheville NC, 5 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1908==
12 June 1908 Cadillac MI, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
15 November 1908 Okmulgee OK, 4 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1910==
29 August 1910 Willow Patch NV, 6 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas)) Sources unclear on date.
==1911==
17 September 1911 Colorado Springs CO, 6 killed (assault) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
23 October 1911 Braddock ND 7 killed (arson) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1912==
14 March 1912 Hillsville VA, 5 killed (shot) (http://murderpedia.org/male.A/a/allen-floyd.htm)
10 June 1912 Villisca IA, 8 killed (axe) (https://en.wikipedia.org/wiki/Villisca_axe_murders) unsolved
16 September 1912 New York NY, 30 killed (bomb) (http://www.slate.com/blogs/atlas_obscura/2014/09/16/the_1920_wall_st_bombing_a_terrorist_attack_on_new_york.html) unsolved
==1914==
20 March 1914 Oak Park GA 4* killed (axe) (https://en.wikipedia.org/wiki/Oak_Park,_Georgia)
15 August 1914 Spring Green WI, 7 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
15 September 1914 Iowa County WI, 7 killed (multiple) (http://murderpedia.org/male.C/c/carlton-julian.htm)
==1915==
6 March 1915 Brunswick GA, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1916==
6 March 1916 Lawton OK, 8 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
29 April 1916 Holland TX, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1917==
24 November 1917 Milwaukee WI (bomb) 10 killed (bomb) (https://en.wikipedia.org/wiki/Milwaukee_Police_Department_bombing) unsolved
==1919==
29 July 1919 Kimblerly OK, 8 killed (arson) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1920==
22 April 1920 Turtle Lake ND 8 killed (multiple) (http://murderpedia.org/male.L/l/layer-henry.htm)
==1921==
3 September 1921 Ormsby MN, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1923==
14 December 1923 Drew MS, 4 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1925==
4 June 1925 Hamilton OH, 8 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1926==
18 March 1926 Stockton & Galt CA, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
5 December 1926 Farwell TX, 9 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1927==
3 September 1927 Youngstown OH, 8 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
11 February 1927 Utica NY, 7 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
12 December 1927 Hennessey OK, 7 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1928==
22 August 1928 Fairfield CA (multiple) (http://murderpedia.org/male.Y/y/ying-leung.htm)
==1929==
15 February 1929 New York NY, 7 killed (poison) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
30 April 1929 Johnson County KY 4 Killed (shot) (http://jerryconley.com/Documents/1929JohnsonCountyShootout.html)
25 December 1929 Germanton NC, 8* killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1930==
10 February 1930 Schafer ND, 6 killed (shot) (http://murderpedia.org/male.B/b/bannon-charles.htm)
6 May 1930 Columbus OH, 8 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1931==
23 June 1931 Quiet Dell WV, 4 killed (1 more not qualifying) (various) (https://en.wikipedia.org/wiki/Harry_Powers) (http://maamodt.asp.radford.edu/Psyc%20405/serial%20killers/Powers,%20Harry.pdf)
25 October 1931 Wagener SC, 6 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1932==
24 November 1932 Seattle WA, 6 killed (assault) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
==1933==
6 March 1933 Cleveland OH, 5 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(Americas))
10 October 1933 over Chesterton IN, 7 killed (bomb) (https://en.wikipedia.org/wiki/1933_United_Airlines_Boeing_247_mid-air_explosion) unsolved
==1935==
8 January 1935 Mount Vernon KY, 8 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1936==
23 January 1936 Danville IL, 7 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
9 July 1936 Indiana Harbor IN, 9 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
16 October 1936 Elko NV, 4 killed (shot) (http://murderpedia.org/male.J/j/jones-luther.htm)
==1937==
23 January 1936 Danville IL, 7 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
28 August 1937 Dennison IA, 7 killed (multiple) (http://murderpedia.org/female.N/n/nollen-elsie.htm)
31 October 1937 Norwalk IA, 6 killed (shot) (http://murderpedia.org/female.M/m/mcanich.htm)
==1938==
16 March 1938 Center TX, 6 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1939==
12 August 1939 Near Harney NV 24 killed (sabotage of a railroad tracks) (https://en.wikipedia.org/wiki/1939_City_of_San_Francisco_derailment) unsolved
==1940==
6 May 1940 Pasadena CA, 5 killed (shot) (https://en.wikipedia.org/wiki/List_of_school_shootings_in_the_United_States) first ‘real’ school shooting.
10 July 1940 New York NY, 7 killed (poison) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1941==
24 April 1941 Irvington AL, 6 killed (arson) (http://www.al.com/news/birmingham/index.ssf/2016/08/alabama_mass_murders_56_killed.html)
==1942==
16 January 1942 Plum City WI, 7 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
2 November 1942 Chatsworth GA, 6 killed (multiple) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1943==
17 May 1943 Crossville AL, 6 killed (shot) (https://en.wikipedia.org/wiki/List_of_rampage_killers_(familicides_in_the_United_States))
==1944==
28 March 1944 San Francisco CA, 22 killed (arson) (http://murderpedia.org/male.H/h/holman-george.htm)
==1945==
8 July 1945 Salina UT 9 killed (shot) (https://en.wikipedia.org/wiki/Utah_prisoner_of_war_massacre)
==1948==
6 November 1948 Chester PA, 9* killed (shot) (http://murderpedia.org/male.C/c/collins-melvin.htm)
==1949==
6 September 1949 Camden NJ, 13 killed (shot) (http://murderpedia.org/male.U/u/unruh-howard.htm)
there doesn't seem to be anything here