POLICE SEARCHING the underground dungeon where Austrian pensioner Josef Fritzl held his daughter captive for 24 years are to use sonar equipment in case there are more hiding places.

Officers can work only one hour at a time in the search because of a severe lack of oxygen, the officer in charge of the investigation in Amstetten said.

Austrian Police Colonel Franz Polzer said the entrance to the windowless rooms where Fritzl, 73, held his daughter captive, fathering seven children by her, was protected by two steel doors that locked electronically.

"They are open, but we are trying to get another way out of this room because the working conditions in this prison are so exceptional," Polzer said.

"Investigators wearing special clothes and masks ... can work there only one hour and during this hour they try, one team after the other, to gather everything available in this living space. They search particularly for DNA traces to establish if the alleged criminal really committed this on his own.

"Not until then can we start with technical investigations like sonar probes, cavity and sound measurements, and also to comprehend all the electric and electronic systems."

Leopold Etz, chief of murder investigations for Lower Austria province, said investigators were questioning more than 100 people who lived in Fritzl's house during the time of his daughter Elisabeth's captivity as well as others who said they knew him.

"We're casting a wide net. ... It's a lot of work," Etz said.

Authorities said they expect to receive Fritzl's old court records early next week that Austrian media say document a 1967 rape allegation.

Police said Fritzl has confessed to fathering seven children with his now 42-year-old daughter Elisabeth, who was locked away when she was 18. They say three of the children were hidden in the dungeon of the Amstetten apartment building and that Fritzl confessed to burning the body of one child after it died in infancy.

Fritzl was an "absolute ruler" in his household and his tyranny caused most of his seven children with his wife Rosemarie to flee the home as soon as they were old enough, chief investigator Col Franz Polzer said.

"He forbade anyone to ask where he was or what he was doing," Polzer said.

Fritzl's children have given investigators consistent reports of daily life in the family with Rosemarie, Polzer said, including Elisabeth who described the difficulties she experienced with her father even before her imprisonment.

Polzer declined to confirm an Austrian media report that Elisabeth ran away from home as a teenager, but noted she may have been sexually abused by him from when she was as young as 12 or 13.

Media reports suggest Fritzl was arrested in the 1960s in Linz and may have served prison time. Police have declined to comment. But one newspaper carried an excerpt of what it said was a 1967 court record found in the state archives in Linz, in which a Josef F was accused of breaking into the apartment of a 24-year-old nurse and raping her.

The daughter of a former employer of Fritzl backed up the reports. "He was hired even though he had a record," said Sigrid Reisinger, who heads the Amstetten construction material firm Zehetner and whose father employed Fritzl there from 1969-71. She said the alleged crime was of a sexual nature but did not recall details.

Lower Austria prosecutor Gerhard Sedlacek says the file of old court records should arrive tomorrow or Tuesday in St Poelten, where Fritzl is being held.

"I do not yet know the contents," Sedlacek said.

Authorities have said that what appears to have been Fritzl's crime came to their attention on April 19, when Elisabeth's eldest daughter was admitted to a hospital suffering from an unidentified infection.

Doctors appealed on TV for her mother to come forward with information about the girl's medical history. Fritzl accompanied Elisabeth to the hospital a week later.

Elisabeth's three other children by Fritzl - a son and two daughters - were removed from the cellar by him as babies, police said. Fritzl and his wife, who was told Elisabeth had abandoned the children, adopted one and had effective custody of the other two.

Fritzl faces up to 15 years in prison if convicted on rape charges. Prosecutors were investigating whether he can be charged with "murder through failure to act" in connection with the baby's death, punishable by up to 20 years in jail.