Amstetten, Austria - An Austrian man who imprisoned his daughter for 24 years and fathered seven children with her had threatened to kill his victims by pumping gas into the cellar where he was holding them, police said on Thursday.

Police were also investigating a claim by Josef Fritzl that the locked door to the cellar would open automatically if he was away for an extended period.

Fritzl, 73, told police earlier this week he had made threats to pump gas into the cellar where he hid his daughter and three of her children, a spokesman for the national criminal investigation office said.

"But we have not yet proved if this claim is correct," Helmut Greiner of the Federal Criminal Police Office said.

Greiner said investigators were also examining the reinforced concrete door to the cellar.

"They are examining how its mechanism works, Fritzl has said the door would open automatically after a certain time if he was away," Greiner said.

Fritzl has now stopped talking to police on the advice of his lawyer, according to officials.

The doctor treating the family said that after up to 24 years without sunlight in a cramped bunker, they were gradually getting back to normal.

"It is a question of restoring their spatial orientation step by step. We are convinced this will succeed in the next few weeks," Doctor Berthold Kepplinger said in a statement.

DNA tests have confirmed that Fritzl was the father of all six of his daughter's surviving children and prosecutors are probing him for rape, incest, coercion and the death of the seventh child, whose remains he burnt in a furnace.

Their 19-year-old daughter is critically ill in hospital.

The small Austrian town of Amstetten where Fritzl lived has pleaded for the family to be left alone and for life to return to normal.

The clinic where the family are being treated said a Belgian television camera crew had tried to get into the building to film the patients but had been removed by security staff.

"We are also having problems with photographers. They are sitting all over the place. People are hassling our staff. It just goes on and on," clinic spokesman Klaus Schwertner said.

"Our first priority is to protect the family from being disturbed so they can try to start their new life," he added.

Local officials said on Wednesday they would be holding no more regular news conferences and said 73-year-old Josef Fritzl's victims would not be appearing in public. They appealed to journalists to leave the family alone.

In his first public reaction on Wednesday, Chancellor Alfred Gusenbauer expressed concern for the victims and announced a campaign to repair Austria's image abroad. He emphasized on Thursday the case was not Austria's fault but Josef Fritzl's.

Additional reporting by Paul Bolding in Vienna