What is meaning of unknown??

Read this excerpt from we’ve got a job: the 1963 children’s march. though connor was commissioner of public safety, blacks knew that it wasn't their health and safety he intended to protect. and he certainly didn't put out their fires. between the late 1940s and early 1960s, more than fifty black homes and churches in birmingham were bombed. one neighborhood was hit so often, it was called "dynamite hill." no one was ever prosecuted, even when the police could identify the bombers. what is the tone of the excerpt? inquisitive sentimental apologetic scornful