Article L741-6 of the French Code governing the entry and residence of foreign nationals and the right of asylum

The decision to place the foreign national in detention is taken by the administrative authority, after the foreign national has been questioned or, where applicable, during his detention for the purposes of verifying his right to move or reside, at the end of his police custody, or at the end of his period of incarceration in the case of detention. It shall be in writing and state the reasons on which it is based.
It shall take effect from the date of its notification.

Original in French 🇫🇷

Article L741-6

La décision de placement en rétention est prise par l’autorité administrative, après l’interpellation de l’étranger ou, le cas échéant, lors de sa retenue aux fins de vérification de son droit de circulation ou de séjour, à l’expiration de sa garde à vue, ou à l’issue de sa période d’incarcération en cas de détention. Elle est écrite et motivée.
Elle prend effet à compter de sa notification.