August 20, 1986 (well before Columbine), Edmond Post office sorting facility.
A man entered, carrying 3 semi-automatic pistols, and sought out the supervisors who had verbally disciplined him the previous day. He found one of them (the other came to work late that day) and shot him. According to Wikipedia, 100 workers occupied the small facility at the time of the attack. Fourteen people died at the scene, and six others received wounds requiring hospitalization. The day's violence ended when the perpetrator shot himself in the forehead.
Patricia Ann Chambers, 41, part-time clerk
Judy Stephens Denney, 41, part-time clerk
Richard C. Esser Jr., 38, supervisor
Patricia A. Gabbard, 47, clerk
Jonna Gragert Hamilton, 30, clerk
Patty Jean Husband, 48, supervisor
Betty Ann Jarred, 34, clerk
William F. Miller, 30, rural carrier
Kenneth W. Morey, 49, rural carrier
Leroy Orrin Phillips, 42, rural carrier
Jerry Ralph Pyle, 51, rural carrier
Paul Michael Rockne, 33, letter carrier
Thomas Wade Shader Jr., 31, part-time clerk
Patti Lou Welch, 27, clerk
This incident may, unfortunately, be the origin of the phrase "going postal". (According to Wikipedia).