From media I learn that many of the people mentioned related to shooting are known by the police. What does it mean? Both perpetrators and victims are often named as known by the police.

I understand, that the perpetrators are not known as guilty of the killings. So was it with Al Capone. Finally the police could get him as guilty of tax fraud.

The police says that it is often important to get these individuals off the street. Even a small quantity of drugs could give them prison. So why are they just known but never guilty?