The preliminary investigation must not irremediably compromise the rights of the defense. Therefore, investigators are subject to a duty of fairness or “loyalty” (loyauté) when seeking evidence. To this end, they must clearly inform undertakings of the purpose of their investigation. Indeed, whatever the legal framework (Art. L. 450-3 or L. 450-4 of the Commercial Code), the persons questioned cannot be led to make statements the scope of which they do not understand and which could be used against them. The duty of fairness refers to the right not to be compelled to testify against oneself or not to confess guilt contained in the International Covenant on Civil and Political Rights. Thus, a person’s acknowledgement of his or her participation in an anticompetitive practice is meaningless if the object of the investigation has not been clearly communicated to him or her.

It is necessary and sufficient that the object of the referral be communicated in order for the duty of fairness to be fulfilled. There is no requirement either on the rapporteurs or on the investigators to pose the same questions to all the operators  in a sector and to communicate the answers of third parties or to submit the letter of referral of the Minister at a hearing. Nor is the use of spontaneously communicated documents contrary to the principle of fairness. Likewise, respect for the principle does not prevent the person suspected of an offense from being heard, since a request for explanations during the investigation cannot be equated with an interrogation. Persons may even be heard as witnesses, not as persons accused of an offense, without being informed of the object of the investigation. Investigators may also request disclosure of documents in the course of their investigations, provided that the request for disclosure is neither general nor unspecific and identifies the documents concerned and the period under investigation. On the other hand, recordings of telephone calls obtained unfairly, without the knowledge of the person having made the remarks, cannot be used as evidence, even if they have been subject to the adversarial principle . Nevertheless, the transcription of a telephone message left on a company’s answering machine has been held as not unfair. Finally, an authorization (warrant) must be annulled when it is based on stolen computer data or on documents disclosed in compliance with orders subsequently retracted and obtained unfairly.