Crime statistics can be found online at the federal, state, and local levels. These statistics can be used to back up your argument in a research paper, to show change, or to show the effects of a certain policy or program. It is pretty easy to find crime statistics at the federal level.
The Bureau of Justice Statistics and the Uniform Crime Report are great online tools for retrieving that sort of information.
Each state has its own Uniform Crime Report (UCR) that offers detailed statistics on various crimes in the state.