Police say Fritzl has admitted imprisoning and raping his daughter
The Austrian man who imprisoned his daughter in a cellar for 24 years and sexually abused her may be charged with slavery, Austrian prosecutors say.
The slavery charge, which could bring a 20-year jail sentence, is being considered by state prosecutors in St Poelten, their spokesman said.
Josef Fritzl has reportedly confessed that he fathered seven children by his daughter Elisabeth, who is now aged 42.
His alleged victims are undergoing treatment at a psychiatric hospital.
Prosecutors' spokesman Gerhard Sedlacek, quoted by the Austrian ORF news website, said nobody had been prosecuted for the crime of slavery yet under Austrian law.
Possible murder charge
Mr Sedlacek said formal charges against Mr Fritzl should be ready by the end of September and he may go on trial later this year.
Mr Fritzl is being held in pre-trial detention in St Poelten, 80km (50 miles) west of the capital, Vienna. Police found a sophisticated secret dungeon at his home in the quiet town of Amstetten in April.
Prosecutors are still trying to determine how one of the seven babies Elisabeth was forced to bear by her father had died.
Mr Fritzl confessed to incinerating the body of the dead child, officials say. He might face a murder charge for the baby's death.
Three of Elisabeth's surviving children were confined to the cellar and three were raised above ground, officials say.
DNA tests have shown he is the father of all six.
The case first came to light in April after 19-year-old Kerstin, one of the children fathered by Mr Fritzl, became seriously ill and was taken to hospital.