Question: Several months ago I was informed by someone I know about very serious crimes they were involved with. My question is what does Jewish law say about this situation? Do I just inform a rabbi so my conscience is clear, or do I report him to the authorities?
Answer: Judaism believes that each of us is responsible for the consequences, positive or egative, of our actions. Jewish civil and criminal law embraces the principle of "Dina D'malkhuta Dina," we are obligated to follow the law of the land in which we live. If you know of someone who have committed a serious - and possibly ongoing - crimes, then by not reporting the information you become a party to the crime - you become in part responsible for their actions. If you have heard rumors or gossip or second/third hand reports, then you are not obligated to follow up -- such would be lashon hara or rekhilut, talebearing. However, if you have first hand knowledge of serious crimes, you ought to go to the police (not just to your rabbi).