Why was Emmett Till's murder and the "not guilty" verdict of his killers were such a spark for the Civil Rights Movement?

Emmett Till's murder was such a spark in the civil rights movement because there was proof that Emmett was not guilty of what the people accused him of, he was only 14 years old at the time of his death and because the people that killed him admitted in a national newspaper that they killed him and were still found not guilty.