powerful. >> it was social media that propelled the black lives matter movement onto a global stage. >> put your hands behind your back. >> i can't breathe. >> keep your hands where they are, please. >> yes, i will, sir. i'll keep my hands where they are. >> reporter: when cell phone images of unarmed african-americans killed by police went viral. >> the body of michael brown lays on that ground for 4 1/2 hours shocked america back its consciousness once again. it woke so many people up that thought emmett till was a figment of the past and not a very relevant figure of the present. emmett till is michael brown and tamir rice and sandra bland and eric garner, and all of those folks because they woke us up once again.