(CNN) On May 4, 1970, 13 seconds of gunfire seemed to bring America to a halt.

On the Midwestern campus of Kent State University, four students had been killed and nine others were injured when Ohio National Guard members opened fire on demonstrators protesting the Guard's presence and the expansion of the Vietnam War.

The event so shocked the nation that more than 500 colleges were shut down as students responded to the killings with outraged protest. Life magazine and Newsweek dedicated cover stories to the incident, with Newsweek and The New York Times famously showcasing the now-iconic photograph of a young woman screaming as she knelt over the body of a Kent State student.

Guardsmen patrol the empty Kent State University campus on May 6, 1970. The school was immediately closed and a strike involving more than 4 million students nationwide followed.

Guardsmen patrol the empty Kent State University campus on May 6, 1970. The school was immediately closed and a strike involving more than 4 million students nationwide followed.

Students run for cover after the National Guard opens fire. Twenty-eight guardsmen fired into the crowd for 13 seconds, wounding nine students and killing 4.

Students run for cover after the National Guard opens fire. Twenty-eight guardsmen fired into the crowd for 13 seconds, wounding nine students and killing 4.

A student throws a tear gas canister back at National Guardsmen. After several standoffs, the troops headed back up a hill in the direction of the ROTC building. As they reached the top, they turned toward the demonstrators and opened fire.

A student throws a tear gas canister back at National Guardsmen. After several standoffs, the troops headed back up a hill in the direction of the ROTC building. As they reached the top, they turned toward the demonstrators and opened fire.

The Ohio National Guard is called in to disperse a rally scheduled for at noon on May 4, 1970. Shortly after the protest began, guardsmen fired tear gas at the students. Some students said they were surprised the guardsmen followed them as they ran away from the tear gas. The guardsmen were clearly armed, but many students later said they believed their weapons were not loaded with actual ammunition.

The Ohio National Guard is called in to disperse a rally scheduled for at noon on May 4, 1970. Shortly after the protest began, guardsmen fired tear gas at the students. Some students said they were surprised the guardsmen followed them as they ran away from the tear gas. The guardsmen were clearly armed, but many students later said they believed their weapons were not loaded with actual ammunition.

The burning of the ROTC building brings the Ohio National Guard to the campus. Some students described their presence as "frightening," and called it a military takeover. Others said the guardsmen were congenial and chatted with the students, with no sense or feeling that violence would soon overtake the grounds.

The burning of the ROTC building brings the Ohio National Guard to the campus. Some students described their presence as "frightening," and called it a military takeover. Others said the guardsmen were congenial and chatted with the students, with no sense or feeling that violence would soon overtake the grounds.

A night of violence in downtown Kent is followed by a student march to the campus ROTC building the next day. Some students try to burn the building down. While the protesters claim they left the building intact and in the hands of campus police when they returned to their dorms, the building is destroyed. It is still not clear who burned it down.

A night of violence in downtown Kent is followed by a student march to the campus ROTC building the next day. Some students try to burn the building down. While the protesters claim they left the building intact and in the hands of campus police when they returned to their dorms, the building is destroyed. It is still not clear who burned it down.

On Friday, May 1, 1970, students at Kent State stage a protest on campus, the first in a series of protests.

On Friday, May 1, 1970, students at Kent State stage a protest on campus, the first in a series of protests.

President Richard Nixon addresses the nation in April 1970 to explain the expansion of the Vietnam War into Cambodia. Anti-war activists all over the country, including at Kent State, saw this as a betrayal by the President, who promised to end the war when he was elected fewer than two years earlier.

President Richard Nixon addresses the nation in April 1970 to explain the expansion of the Vietnam War into Cambodia. Anti-war activists all over the country, including at Kent State, saw this as a betrayal by the President, who promised to end the war when he was elected fewer than two years earlier.

Four students died and nine others were wounded on May 4, 1970, when members of the Ohio National Guard opened fire on students protesting the Vietnam War at Kent State University in Ohio. In this Pulitzer Prize-winning photo, taken by Kent State photojournalism student John Filo, Mary Ann Vecchio can be seen screaming as she kneels by the body of a slain student.

Four students died and nine others were wounded on May 4, 1970, when members of the Ohio National Guard opened fire on students protesting the Vietnam War at Kent State University in Ohio. In this Pulitzer Prize-winning photo, taken by Kent State photojournalism student John Filo, Mary Ann Vecchio can be seen screaming as she kneels by the body of a slain student.

"Is dissent a crime?" the father of one of the slain students, 19-year-old Allison Krause, asked in Newsweek. "Is this such a reason for killing her? Have we come to such a state in this country that a young girl has to be shot because she disagrees deeply with the actions of her government?"

It's a question that likely weighed heavily on the minds of another group of parents farther South -- ones who'd also lost their children on college campuses in an eerily similar way.

Just 11 days after the deadly shooting in Ohio, two students were killed and 12 were wounded when police fired more than 100 rounds of bullets on protesters gathered at Mississippi's predominantly black Jackson State College.

And two years earlier, in 1968, three students were killed by authorities during protests against segregation at South Carolina State in Orangeburg, another historically black institution.

But there weren't national news magazine covers, Pulitzer Prize-winning photographs or popular songs memorializing these deaths, as Crosby, Stills, Nash & Young's "Ohio" did for Kent State.

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For those who lived through the time, the reason for the lack of coverage on the other campus shootings is pretty simple: "Kent State was four white students in Ohio," said Gene Young, a former Jackson State professor, when asked by NPR why the tragedies at Jackson State and South Carolina State aren't as prominent in the nation's memory.

"Jackson State and Orangeburg were black colleges in the South," Young continued. "Two black students on a black college campus in Mississippi that had the history of Emmett Till, Medgar Evers, Chaney, Goodman and Schwerner. It was just another day of business as usual, racist law enforcement officials victimizing black people in Mississippi."

In recent years, documentarians, historians and others have worked to rectify the larger public's obliviousness to what happened at Jackson State and South Carolina State, ensuring that the students who lost their lives in protest, like those at Kent State, wouldn't be forgotten.

'Orangeburg Massacre' at South Carolina State

It started with a bowling alley.

In February 1968, students from historically black South Carolina State were protesting segregation at All Star Bowling lanes, the only bowling alley in Orangeburg.

On February 6, the first night of protest, students entered the bowling alley and were denied service, as one student participant recalled to USA Today . They went back a second night, and the tension began to reach a boiling point.

By the third night, on February 8, the student protest against segregation had moved back to campus, where it was later met with violence, as Jack Bass and Jack Nelson chronicle in their book on the incident, "The Orangeburg Massacre."

National Guardsmen with rifles and fixed bayonets march double-time on February 8, 1968, outside the main entrance to South Carolina State.

According to Bass's account , firemen arrived on campus to put out a bonfire erected by students, and state troopers were present to protect the firefighters.

After a tossed banister rail struck one state trooper in the face, 66 armed members of law enforcement lined up around the edge of campus and opened fire.

"Students fled in panic or dived for cover," Bass writes, "many getting shot in their backs and sides and even the soles of their feet."

Eight of the nearly 70 state troopers present that night later told the FBI that they fired their weapons after hearing shots.

By the end, after roughly 10 seconds of gunfire, nearly 30 students were injured and three were dead: Henry Smith, a South Carolina State sophomore; Samuel Hammond, a freshman; and Delano Middleton, a high school student whose mother worked at the school.

South Carolina Highway Patrol officers watch over two injured students at the campus in Orangeburg.

"South Carolina State was the first time ever in the history of America that a college student had been killed on their campus for doing absolutely nothing," remarked Cleveland Sellers, a civil rights activist who attended South Carolina State and was involved in the February protests, at a conference on the incident in 2012.

But "unlike Kent State," notes journalist Bass, "the students killed at Orangeburg were black, and the shooting occurred at night, leaving no compelling TV images.

"What happened barely penetrated the nation's consciousness."

30 seconds of gunfire at Jackson State

For its part, Kent State University has made an effort to remember the student lives lost at Jackson State in the wake of its own tragedy.

Nevertheless, another campus shooting in May 1970 has still gone largely unnoticed.

Even before the shootings, Jackson State students had been targeted with harassment and other acts of violence by whites who lived in the area. According to former JSU professor Young, "motorists would drive through the campus making racist (epithets), making (sexual) innuendos against some of the black female students on that campus."

Sen. Birch Bayh, D-Indiana, passes demonstrators protesting the slayings of two African-Americans on the campus of Jackson State College in May 1970.

On the night of May 14, The New York Times reported, bottles and rocks were thrown at white drivers passing through -- actions Jackson State students attributed to non-students on campus. A rumor that Charles Evers, mayor of a nearby town and the brother of slain civil rights leader Medgar Evers, had been killed only added to the unrest. By midnight, armed local and state police had arrived at Jackson State.

"Things just came to a head when law enforcement officials marched onto the campus in front of Alexander Hall women's dormitory," Young recalled. "Shortly after midnight, a bottle broke on the pavement and law enforcement officials fired over 200 rounds of bullets into a women's dormitory from the bottom floor to the top floor."

The shots lasted 30 seconds, and a 1970 report from the President's Commission on Campus Unrest found that about 400 bullets or pieces of buckshot had been fired into the women's dorm, where witnesses said roughly 100 students were gathered.

The bullet-riddled windows of Alexander Hall, a women's dormitory at Jackson State College.

According to The New York Times , police said they were responding to sniper fire; the federal investigation didn't find evidence of anyone shooting from the locations police targeted with their weapons.

There were reports of students shouting angrily at the officers and throwing rocks, as Evers acknowledged in a telegram to President Nixon. "I am not saying they were right for throwing rocks," Evers said in his telegram, according to the Times. "But rocks didn't warrant coming in and shooting. They (the police) came out to kill."

Two students died that night: 17-year-old James Earl Green, who was in high school, and 21-year-old Phillip L. Gibbs, a junior at Jackson State and the father of an 18-month-old.

Today, Jackson State, now a university, has the Gibbs-Green Monument, which the school describes as "a permanent memorial to the slain students and a tangible reminder to all students that the Jackson State Tragedy must never be forgotten."