A police officer carries a cardboard box out of the house of Josef Fritzl, in Amstetten yesterday. (AFP)

VIENNA • The lead detective in the Austrian "house of horrors" case said yesterday that forensic staff are struggling for air in the dungeon where Josef Fritzl hid his offspring for 24 years, according to the local news agency.

Officers wearing surgeon's masks to protect the crime scene's integrity are having to come up regularly for fresh oxygen, Franz Polzer told APA newsagency, with specialist psychologists on hand to help his team cope in case of

trauma.

"The work in the cellar is overwhelming and oppressive for the investigators," Polzer is quoted as saying. "Every object reminds them of what went on here."

DNA tests have backed up a confession Fritzl gave police, the full details of which have yet to be made public, but officers are still trying to ascertain exact living conditions for his captive daughter Elisabeth and their surviving children born or raised underground.

Work in the cellar-where investigators found not just one, but two heavy steel doors which could only be opened with a remote control device-is expected to take weeks, Polzer has already stated. Attempts to ascertain what police said this week was Fritzl's threat to gas the occupants of his windowless bunker if anything happened to him, according to Elisabeth's account, are high on their list of priorities.

Officers are also searching for additional hidden chambers in Fritzl's warren, which a German magazine due out tomorrow, Der Spiegel, says comprised just one room for the first nine years of Elisabeth's imprisonment, citing police.

That would imply that incestuous sex or rape and manslaughter over the death of a baby twin-all of which prosecutors have listed among possible charges-took place in the presence of young children.

From 1984 to 1993, the report says, "repeated rapes committed by Josef Fritzl" took place in front of children born in 1988, 1990 and 1992, again citing Elisabeth's testimony to police.

The Der Spiegel report, details of which were trailed by the magazine today, again cites investigator reports to state that Elisabeth was chained to a post with handcuffs for the first two days underground.

It says Fritzl then tied his daughter, 19 at the time, to a leash for the next six months, "or perhaps nine ... so that she could at least go to the toilet (on her own)." Spiegl says Fritzl has denied these accusations during ongoing questioning.

The magazine says Elisabeth has exonerated her mother Rosemarie, the 73-year-old Fritzl's wife, who already bore seven children of her own by him.

The captive "has clearly vindicated her mother during her police interview," the magazine says in a lengthy journalistic investigation. The 42-year-old, currently receiving medical and psychological treatment in common with all of her children, spelled out that Rosemarie "knew nothing of her incarceration, and had nothing to do with (Elisabeth's plight)."

"Only her father, and no-one else, provided her with food and clothes," the report adds.

Meanwhile the town of Amstetten yesterday asked its 23,000 inhabitants to come up with a series of messages to purge itself of potentially confused feelings since the scandal first broke last weekend. Mayor Herbert Katzengruber said that the events of the past week "have left us all dumbstruck" and asked for residents to contribute words, drawings or simply signatures to 35-metre (yard) banners which will go on display on Wednesday.

The town is organising a demonstration at its main square whose theme is to "restore confidence" in Amstetten. Katzengruber added that the family, "if it wants," can count on the town's full support to "re-start" living again.