Austria ? The Austrian man accused of imprisoning his daughter for 24 years and fathering her seven children planned to build his secret cell as early as 1978 when his daughter was 12, authorities said Monday.

Eight doors fitted with sophisticated locks and electronics secured the underground warren of windowless rooms where Josef Fritzl held his daughter captive starting when she was 18. The main door weighed about half a ton, said police Col. Franz Polzer, who is overseeing the investigation.

“This was not built from one day to the next,” Polzer said.

An investigation showed Fritzl applied for permits in 1978 to expand the apartment complex he owned, which was built in 1890, Polzer said. Police believe the expansion plans included the secret rooms, he said.