A girl, 8, was rescued in Switzerland on Sunday, five days after being kidnapped from her grandmother's French home in a "military-style" operation with the alleged involvement of her mother.
After an intensive search, investigators found the girl, Mia, and her mother, Lola Montemaggi, in a squat inside an abandoned factory in the Swiss municipality of Sainte-Croix, French prosecutors said.
Ms Montemaggi, 28, was arrested along with five others accused of helping her.
Three of the men posed as child welfare officials, with forged identifications, to convince Mia's maternal grandmother to hand her over at their home in the village of Poulieres, near France's border with Switzerland, on Tuesday.
No violence was used in the abduction, but the Public Prosecutor of Nancy, Francois Perain, said it was like a "military operation", with the "extremely well-prepared" kidnappers giving it the codename Operation Lima.
They had walkie-talkies, camping gear, fake licence plates and a budget of €3,000 ($3,590) to cover expenses, the prosecutor said.
The kidnappers were not known to police but were described as sharing a "community of ideas".
"They are against the state and mobilised against what they call a health dictatorship," Mr Perain said.
He said the suspects believed that "children in care are unfairly taken from their parents".
After the kidnapping, three of the men and the mother walked over the French-Swiss border, taking turns to be with the child.
Then a man nicknamed Romeo picked up Mia and Ms Montemaggi in a Porsche and drove them to a Swiss hotel.
They spent a night with a woman who was a "sympathiser of the movement" before arriving in Sainte-Croix.
Five people linked to the kidnapping, aged 23 to 60, were arrested in France between Wednesday and Friday.
Mia is safe and in good health, and a psychologist and social worker would take care of her before she was handed back to her grandmother, the prosecutor said.
But with the story becoming big news in France, intense media pressure meant they would not immediately be reunited in Poulieres, investigators said.
Ms Montemaggi did not resist arrest when Swiss investigators arrived at the abandoned factory in two vans, although Mia screamed, AFP reported.
She was taken into Swiss police custody and was expected to soon be the subject of a European arrest warrant for her extradition to France.
Nearly 200 police officers were part of the search effort.
Her grandparents said her rescue was "a huge relief".
"It is the end of nights of anguish and fear for the life of our little girl, in particular because of the extremist commitments of the kidnappers," they said.
