There must be more than that. The Charles Manson murders were 1969.
I start here. 1949.
https://www.smithsonianmag.com/history/story-first-mass-murder-us-history-180956927/
Howard Unruh, the walk of death. 1949
Richard Speck, killed eight student nurses with a knife. 1966
Charles Whitman, The texas Tower shooter. 1966.
Manson had others do his dirty work.