Does anyone know what real life case this is based on?

I am from that country and I am almost certain a lot of this was based on Marc Dutroux.
He actually kidnapped and abused six girls and murdered four of them. Sabine was the fifth, and when he kidnapped yet another he was careless and this is how he got caught, saving the last two girls.
He also told them that there was a huge network with chiefs who were a lot meaner than him and against whom they had to be "protected".

However, he wasn't dumb enough to kidnap girls with ties between them (he went all over the country).

I think it's the Colleen Stan case (not sure why that website changed her name to Carol Smith).

In the film, Owen Teale keeps talking about "the company" - that's how Hooker managed to control Stan/Smith - he told her that if she ran away the company would torture and kill her family.