There are web sites devoted to this subject, among which are this one and this one.
The wikipedia article says,
The Kent State shootings, also known as the May 4 massacre or Kent State massacre, occurred at Kent State University in the city of Kent, Ohio, and involved the shooting of students by members of the Ohio National Guard on Monday, May 4, 1970. Four students were killed and nine others wounded. The students were protesting the American invasion of Cambodia which President Richard Nixon launched on April 25, and announced in a television address five days later.