There is no internet (let alone social media or body cams) during that era, since Jim Crow in America was still being enforced towards black people at that time, but the civil rights movement is gaining traction (since Martin Luther King was alive). I mean, cop violence has been around for a long time before body cams were invented. How did people hear first hand encounters from those victimized back then without reliance on social media?
For a big part, they didn’t. Of course in smaller circles people knew exactly what happened, but nationally it was denied or downplayed and covered up by the South. Until sometimes they couldn’t. Like in the Emmett Till case, who was on a family visit from Chicago and was lynched in Alabama. This got a lot of media coverage nation wide for a change.