Well yes. But no. And that's exactly why there is always a risk of a "politically tainted" investigations. The "Juge d'instruction" is not an independent judge

Well yes. But no. And that's exactly why there is always a risk of a "politically tainted" investigations. The "Juge d'instruction" is not an independent judge that will, out of his own will, start an investigation. He can start an investigation when asked by the "procureur", directly or indirectly under influence of the executive power, or by private citizens, as a "partie civile". The Sarkozy case was started by the former. On top of that, the "juge d'instruction" is nominated by the Minister of Justice for a period of 3 years, which means it is, once again, linked to the executive power.