Mr Fritzl refuses to explain his actions
Forensic work in the dungeon where an Austrian man allegedly held his daughter for 24 years is "oppressive", police chief Franz Polzer has said.
Officers wearing special masks can only work one hour at a time because of the lack of oxygen in the windowless cellar, he told AP news agency.
Police say Josef Fritzl, 73, imprisoned and raped his daughter Elisabeth in the cellar in Amstetten in Lower Austria.
They say she bore his seven children - three of whom never saw daylight.
Mr Fritzl, who is in custody, has confessed to the crime in a written statement, police say.
Mr Fritzl is refusing to answer any more questions, as police try to piece together conditions in the cellar - as well as the suspect's life.
Investigators say they have found two heavily reinforced concrete doors that could only be opened by remote control.
Mr Polzer told AP: "They are open now, but we are trying to get another way out of this room because the working conditions in this prison are so exceptional."
Officers are expected to comb the scene for several weeks
He added that officers could "work there only one hour and during this hour they try, one team after the other, to gather everything available in this living space".
Mr Fritzl also reportedly told his victims they would be gassed if anything happened to him.
Technicians are trying to establish if this was more than a threat.
Work in the cellar is expected to take weeks.
In a separate interview with Austria's APA news agency, he said the work was "overwhelming and oppressive for investigators".
Mr Fritzl's alleged crimes came to light when Elisabeth's eldest daughter Kerstin, 19, became seriously ill.
She was allowed out of the cellar and admitted to hospital in Amstetten.
Police then issued an appeal to Elisabeth Fritzl to contact them about her daughter, and later picked up Mr Fritzl and Elisabeth near the hospital.
Elisabeth and the children are now in care with the Austrian authorities, who are protecting their privacy at a psychiatric clinic.
Kerstin is fighting for her life in hospital.