AMSTETTEN, AUSTRIA - An Austrian man came up with the idea for a windowless warren under his apartment building years before locking his daughter in a cell secured by sophisticated electronics, locks and a half-ton door, authorities said Monday.

Josef Fritzl may have been plotting the design of the basement dungeon six years before authorities say he took his daughter Elisabeth captive in 1984 when she was 18, investigators said. He is accused of detaining and raping her for 24 years, and fathering seven children with her.

"We can't just look back to 1984," police Col. Franz Polzer said. "The logic says the idea was already there, or an obsessive thought played a role, to build this jail, dig out these rooms and later to equip them and imprison the daughter there."

Local building authorities by 1978 had approved expansion plans for the apartment building Fritzl owned in Amstetten, 75 miles west of Vienna.

"We are working with certainty on the idea that already in the planning phase he had the intention to build a small space, a small secret, a small dungeon unknown to the building authorities," Polzer said.

The half-ton door guarded the main entry. Investigators uncovered a second entry involving multiple doors, including one made of steel and protected by an electronic code, he added.