(CNN) -- An 80-year old Austrian man has been taken into custody for allegedly sexually abusing his wife and his two mentally disabled daughters for more than 40 years, police and a prosecutor said Thursday.

"We fear that he might bother his victims or continue to abuse women," chief prosecutor Alois Ebner, who ordered the arrest, told CNN.

The man's wife died three years ago, investigators said, but the daughters still lived with their father in the northwestern town of Braunau. They slept on a wooden bench and were threatened by their father with a pitchfork, authorities said.

"He isolated the two women from the outside world for all the time," said Ernestine Heger, another prosecutor in the case.

Police freed the two sisters in May 2011, after a social worker brought their allegations to authorities. The man had fallen on the floor that month and could not get up, officials said, and his daughters refused to help him for two days. The women then entrusted themselves to a social worker, who informed police about the situation.

After being treated in a hospital, the man was transferred to a nursing home in Braunau while the case was being investigated, authorities said.

The two daughters have not yet been questioned by prosecutors. According to Austrian law, they can refuse to testify.

In 2008, Austrian Josef Fritzl was arrested after having imprisoned and raped his daughter for 24 years. He was convicted on charges of incest, rape, false imprisonment, assault and murder and sentenced to life in prison in 2009. The murder charge was for allowing one of the babies he fathered with his daughter to die shortly after birth.

Heger said that in contrast to the Fritzl case, the sisters in Braunau "were not really imprisoned.

"Although they could not move where they wanted, neighbors have seen them several times throughout the years," she explained.

The two sisters are now in psychological care, officials said.