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Freiburg, Germany - The police investigation widened in south-western Germany on Friday after a mother was among eight arrests made in relation to the rape of her 9-year-old son.

Investigators seized film and other recordings and are probing the case for further criminal activity among the paedophile ring, a police spokeswoman told dpa on Friday.

At the centre of the case, in which all the suspects are now in custody, is the 47-year-old mother of the boy and her 37-year-old partner, both residing near the wealthy town of Freiburg.

They are alleged to have sexually abused the boy together and sold him to other men to abuse via the internet.

According to the Public Prosecutor's Office in Freiburg, some of the suspects already have a criminal history of child abuse.

Since the arrests, the boy has been under state care, according to a spokesperson for Child Protective Services.