By Jennifer Hansler, CNN

(CNN) -- To civil rights activist Heather Booth, Dr. Martin Luther King Jr.'s assassination felt like a rupture.

"It was like the breaking of a dream, the breaking of our hopes," Booth recalled in an interview with CNN.

On the evening of April 4, 1968, King was shot and killed on the balcony of the Lorraine Motel in Memphis, Tennessee. He was 39 years old. James Earl Ray pleaded guilty to the assassination in 1969 -- though he later recanted his plea -- and was sentenced to 99 years in prison.

"I just could not believe that anyone could shoot such a person like a deer in the woods," said Bob Zellner, a civil rights organizer who counted King among his mentors. Zellner told CNN he had been working with another famous civil rights leader, Fannie Lou Hamer, in Mississippi when they heard the news.

"It was devastating to all of us," said Zellner, who was the first white field secretary for the Student Nonviolent Coordinating Committee.

The death of the iconic social justice minister felt like more than a tragedy for the organizers who spoke to CNN: It felt like a backlash against the progressive agenda they were working toward.

"We began to see it as a pattern of repression against the advancements of the civil rights movement," Zellner said.

Yet even in the immediate aftermath of King's assassination, some were spurred to action.

Activist Bob Moore said he was having a planning session at the Student Nonviolent Coordinating Committee office he had started in Maryland when he received word of the assassination.

"We immediately switched gears and ran off 50,000 leaflets ... basically the message we were trying to get over was 'Don't mourn, organize,' " he told CNN.

Reactions run the gamut

Moore and his colleagues had planned to call for a day of mourning, but those efforts were cut off by riots that broke out in Baltimore. Violence flared in more than 100 cities across the US in reaction to the news of King's death, leading to more than 40 deaths and extensive property damage, according to the Martin Luther King Jr. Research and Education Institute at Stanford University.

"Around the country, the shock that this man of peace, this man of believing all people had rights to dignity and respect, was killed, (it) was an explosive reaction around the country," Booth said.

"Living in Chicago, I was frightened both personally but also what it meant for a movement that was trying to build for justice, democracy and freedom, and a movement that was very beautiful," she recalled.

What did it mean for the movement? For activists across the country, it certainly did not signify an end. They knew they were armed with the lessons of nonviolent protest and grassroots organizing, and they had seen it work.

"Black lives didn't really matter in '64. Within a year, because people organized, within a year there was a Voting Rights Act," Booth explained. "So I learned a fundamental lesson that if you organize, you can really change the future."

Many shifted their efforts to the anti-war movement, the women's movement and the labor movement, taking up the mantle of King's mission for economic equality.

After the passage of the Voting Rights Act of 1965, King had focused on opposing the Vietnam War and tackling the issues of systemic racism, poverty and unemployment -- cornerstones of his Poor People's Campaign.

"As a social gospel minister he had always wanted to deal with the issue of poverty. That was very clear in his writings from the early 1950s, long before he became a civil rights leader, at least several years before that, before he moved to Montgomery, he was already talking about dealing with unemployment, slums, economic insecurity," Dr. Clayborne Carson, the founding director of Stanford's King Institute, told CNN. "His particular concern as a minister was to deal with the social problems that affected black Americans in the North as well as the South."

Then and now

On that front, activists think there is still work to be done -- and there is concern that the cause has been diluted.

The Rev. Al Sampson, who was ordained by King and worked with him on education and desegregation efforts and as his national housing director, said he thinks parts of King's legacy have been sanitized.

"The whole methodology of who he was and what he was trying to say, I mean his books are not being read like that they should be and they're not being brought into a process," he told CNN. "They're not studying the depth of not so much how he dreamt, but what he understood you had to have in order to handle the weight of the problems you encounter."

"I think what happened was that people who wanted to honor him for his accomplishments chose the route of playing down his goals during the last three years," Carson echoed. "You could see signs of that at his funeral. Many political leaders who attended his funeral would not have wanted to be photographed next to him the day before."

"But what they were honoring was the fame and acclaim he had achieved as the result of his role as a civil rights leader. Then the emphasis had been on his 'I have a dream' speech and his Nobel Prize," he continued. "But I think Martin Luther King made clear that's not what he wanted to be remembered for."

And 50 years after King's death, some said they are still fighting for the progress they made on equal rights. They said they are concerned that President Donald Trump's campaign and election laid bare divisions that they had sought to leave in the past.

"I think a lot of progress has been made. That being said, there are a lot of things which remain kind of the same, namely racism or what we talk about now as white supremacy," Moore told CNN. "So I think those are dangerous things."

"Race is still very powerful in this country and has to be dealt with once and for all," said Zellner, who has volunteered with former Ku Klux Klan members "for 50 or 60 years" and whose own father rejected his role in the KKK and became a civil rights volunteer.

Booth noted that her "involvement still continues even to this day on immigration reform, on voting rights and other efforts expanding our democracy, freedom and justice for all."

Lessons for a new generation

The activists who spoke to CNN seemed eager to impart the lessons they had learned in the 1960s to younger generations.

"We're all living longer," Zellner noted, "so a lot of the people who were working 50 years ago are working now. ... We've trained a lot of organizers since then."

In fact, Zellner said he and his wife were moving from Colorado back to Alabama to "be a part of this new wave of organizing in the Deep South."

Many of the activists applauded modern social movements like Occupy Wall Street, Black Lives Matter and most recently, March for Our Lives.

"I think young people today are taking up the unfinished business of the 1960s," Carson told CNN.

Booth said she thinks that modern organizers "are ready to take action, be engaged, but I think now, there is, by those who want to build a loving society, I think there is a rebuilding of the dream."

Carson said young organizers should remember that they need not wait for an icon like King to make progress.

"For the most part, those movements were sustained by people whose names we don't remember: women, young people, and not unlike people who are living today," he said. "So I think the basic message is: Don't wait for a Martin Luther King to emerge. Create a movement, a great movement, and you'll have great leaders emerge from that. But it doesn't happen the other way around."