VIENNA, May 5 (UPI) -- Austrian authorities say a woman held captive in a cellar for 24 years was forced to aid her father in expanding the prison where she was kept.

Prosecutor Gerhard Sedlacek told a news conference that Joseph Fritzl forced his daughter Elisabeth and some of the children he fathered by her to enlarge their cellar quarters, CNN reported Monday.

"The extra construction was aided by the people living in the cellar by moving earth," Sedlacek said.

Eight individual doors separated the cellar from the main part of the Fritzl home.

Last week Fritzl allegedly confessed to imprisoning his daughter and fathering seven children by her. He also admitted to burning the body of one child who died in infancy.

Prosecutors said they plan to meet with Fritzl for the first time Wednesday.

There are indications Fritzl plans to enter an insanity plea, CNN said.