Where individuals cannot be tricked or entrapped into breaking the law, and are innocent of wrongdoing, law enforcement agents may introduce or plant evidence of law-breaking in the individuals residence, office, automobile, or other property, or in clothing; or they may falsely testify that they found incriminating evidence in the individual's possession. Where the nature of the act does not require physical evidence, agents may conspire to incriminate individuals by giving false witness and perjuring themselves, or by using paid or black-mailed individuals in their place to testify against the innocent. These acts are criminal under all legal systems but occur nevertheless.