France's Génération Identitaire group was joined by cothinkers from other parts of Europe in a carefully orchestrated anti-migrant demonstration on 21 April.
They later claimed to have joined police in patrols of the border with Italy and to have "accompanied" some migrants out of France on the night of 26 April.
On 30 April local authorities claimed to have stopped the Defend Europe patrols, although the right-wingers denied that was the case.
But an inquiry into possible legal action was dropped.
Pro-migrant groups slammed the local prosecutors' office, contrasting the failure to prosecute with legal cases against activists for helping migrants, one of which is due to go to court on 31 May.
After the outcry, on 4 May, the justice ministry issued a circular to prosecutors pointing out that "behaviour hostile to the circulation of migrants" breaches two laws, French media have revealed.
It cites an article of the penal code banning "interfering in the exercise of public service by carrying out actions reserved for a public servant", punishable by up to three years in jail and a fine of 45,000 euros.
Another article forbids behaviour that "creates confusion in the public's perception with the exercise of public service", punishable by up to a year in jail and a 15,000-euro fine.
Prosecutor defends actions
On Friday local public prosecutor Raphaël Balland defended his office, saying that he had ordered an investigation into usurping public authority on 29 April but that no breach of the law has yet been found.
Another inquiry into interfering in the exercise of public service is underway, he said.
NGOs were not impressed.
"We're preparing for the case to be dropped again," Agnès Antoine of the Tous Migrants campaign told Le Monde newspaper. The organisation is looking into the possibility of lodging a civil complaint, she said.