Criminal Investigation in France
Under French legal system criminal investigations are performed by the judicial police under the authority of the Prosecutor of the Republic or a special judge called Investigating Judge (in French "juge d'instruction").
Police investigation usually involves witnesses interviews, scenes analysis, CCTV review, and all other available evidence such as any forensic samples and, of course, suspect interrogatories.
At the end of the investigation, the Prosectutor decides whether there is enough evidence against the identified suspects and, in case there is, whether the suspect should be judged by a criminal court.
For most complex cases requiring thorough investigations, the Prosecutor can ask an Investigating Judge to conduct the investigations.
The investigating judge is an independant judge specialized in criminal investigations with broad powers.
For most serious offenses, qualified as crimes by the French penal code (punishable by prison above 10 years and up to life sentence), the investigation can only be conducted by an Investigating Judge with the assistance of the judicial police.
At the end of his investigation, the Investigating Judge decides whether there is enough evidence to hand over the suspects before a court of law in order for him to be judged.
In case there is not enough evidence the Investigating Judge closes the investigation. If new evidence is found later the investigation can be reopened as long as the crime is not time barred.