Sorry, not well informed about the French judicial system, even if I live there now.
But yes, there is a "juge d'instruction", who is charged with the judicial part of the inquiry, and who will decide whether there is a "case to answer" and whether anybody should be "mis en examen" on specific charges, such as involuntary homicide, gross negligence, or whathever.
seacue,
I think a "juge d'instruction" is indeed not dissimilar to a "district attorney", and it's indeed his decision on whether any charges are to be pressed, and whether there is a case to be put before a court.
CJ