English translation: during the preliminary/pre-trial stage
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Explanation: It's a bit of a mouthful, I admit, but avoids all confusion between common law and civil law terminology. Pre-trial and Preliminary hearings exist in USA law but are not the same thing as under Italian Civil law. By the way, the Istruttoria is now only applied to civil law cases and not in penal cases after the introduction of the Nuovo Processo Penale.
Explanation: Without knowing the context it is difficult to know that exact implication of "in via", but "istruttoria" is an investigation or inquest, and "giudice istruttore" is an investigating magistrate.