From the memorial website - Behind The Tower - that has popped up to recount the tragedy:
On August 1, 1966, a twenty-five year old University of Texas student named Charles Whitman went up to the observation deck of the UT tower armed with guns, ammunition, and canned food. For 96 minutes he held the campus in a state of terror. Whitman killed 14 people that day and wounded more than 30. One of the wounded died a week later and one died decades later of injuries connected with his bullet wounds. Austin Police officers Houston McCoy and Ramiro Martinez (and two other men) made their way to the top of the tower, without knowing who or what they would find. They cornered Whitman and then shot and killed him. Later it was discovered that Whitman had murdered his mother and his wife in the early hours of the morning before his rampage. The shooting was broadcast on the radio and on television and it became a major national and even international news story.
This isn't the first mass shooting in our nation's history. There was actually a mass shooting in Philadelphia back in 1949 that the Smithsonian points to as the first. And if one considers them, we've had massacres and lynchings throughout our history that could fall under this rubric.
But the University of Texas shootings seemed to hit our nation's psyche in the worst way. It was a harbinger of a growing trend towards violent gun deaths in our nation where a singular person could go on a shooting rampage with little reason (well, we've figured out there is a reason: a shitload of anger at the world and access to rifles to start a body count). It happened at a moment of political and cultural turmoil of the 1960s, and added to the chaos of the day.
When we count mass shootings today, 1966 is the starting point, because Whitman's rampage is so memorable and still relatively recent - even at 50 years! - for survivors to still be among us to document the horror. When you ask someone about a campus mass shooting, you may get Virginia Tech or Columbine High or Umpqua Community College if you ask in the Northwest or even Ecole Polytechnique if you ask in Canada. But the odds are pretty good you'll get someone saying "Well there was that one in Texas with the tower, was that in Austin?"
Yeah, it was. And that was 50 years ago. And we've been counting the mass shootings ever since.