Some times the best answer is the most simple. In this case, you just need to know what police agency has jurisdiction over where the crime was committed and call them and make a report. Don't worry about knowing the name of the person you suspect. Just tell the police what you do know. They have to deal with solving cases and identifying suspects all the time.
If you are looking for the name of the person you suspect of having committed a crime, you may have a tougher time finding that answer -- especially if the suspect is a juvenile.