Kent State shootings: Mary Ann Vecchio screams in anguish over the body of Jeffrey Miller after he was fatally shot at Kent State University on May 4, 1970. The photo won a Pulitzer prize for John Filo. (John Filo/Associated Press file)
Kent State shootings: From left, William Schraeder, Allison Krause, Jeffrey Miller, and Sandra Lee Scheuer, were killed at Kent State University on May 4, 1970. (Getty Images)
Kent State shootings: Masked National Guardsmen fired a barrage of tear gas into the rowd of demonstrators at Kent State University. (Getty Images )
Kent State shootings: Members of the Ohio National Guard use tear gas to disperse a crowd of demonstrators at Kent State University. (Getty Images )
50th anniverasry of Kent State University shooting Kent State University student Alan Canfora waves a black flag as Ohio Army National Guardsmen kneel and aim their rifles. (Howard Ruffner/Getty Images )
Kent State shootings: Students kneel on the grass beside wounded classmate John Cleary after he shot when the Ohio National Guard opened fire on antiwar protesters. (Howard Ruffner/Getty Images )
Kent state shootings: Tear gas billows across the Commons at Kent State University on March 4, 1970. (Howard Ruffner/Getty Images )
Kent State shootings: Students react after the Ohio National Guard opened fire at Kent State University on May 4, 1970. (Howard Ruffner/Getty Images )
Kent State shootings: An ambulance leaves the campus of Kent State University after the Ohio National Guard opened fire, killing four students and wounding nine others. (Howard Ruffner/Getty images )
Kent State shootings: View, past the Kent State University Victory Bell (upon which is written "Strike", of a line of Ohio National Guardsman assembled on the Commons, Kent, Ohio, on May 4, 1970. (Howard Ruffner/Getty Images)
Kent State shootings: Ohio National Guardsmen on the Kent State University Commons as they prepare to disperse student protesters, Kent, Ohio, on May 4, 1970. (Howard Ruffner/Getty Images)
Kent State shootings: View of Ohio National Guardsmen in gas masks as they aim their weapons on the Kent State University campus during student antiwar protests. (Howard Ruffner/Getty Images)
Kent State shootings: Teenager Mary Ann Vecchio kneels over the body of Kent State University student Jeffrey Miller who had been shot during an anti-war demonstration on the university campus. (Howard Ruffner/Getty Images)
Kent State shootings: Youths flee as they try to escape exploding tear gar fired into their midst by National Guardsmen on the Kent State University campus. (Bettmann/Bettmann Archive)
Kent State shootings: An Ohio National Guard skirmish line advances up a hill on the campus of Kent State University, on May 4, 1970. (Bettmann/Bettmann Archive)
