The different stages of a criminal investigation, to establish the existence of a violation and whether the charges against the persons charged are sufficient to allow a court judge, should receive, are governed by special rules separate those governing the civil trial. Among these steps, questions the accused occupy a prominent place in the stage of obtaining evidence. On the one hand the active role of judge's direction gives it considerable power in this stage and then, the fragile situation of the accused requires that the power of the judge is well supported. For this reason, each legal system has established certain principles drivers to prevent abuse of this power in criminal matters. These principles are the subject of this study.
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