Alois Lissl, the chief of police of Upper Austria province, said that although no evidence had surfaced so far, police have widened their investigation into the unsolved murder 22 years ago to include the incest suspect.
The bound body of 17-year-old Martina Posch was found on a shore of the Upper Austrian lake of Mondsee in 1986. Josef Fritzl's wife owned part of an inn and camping ground on the other side of the lake at that time.
Fritzl would be asked for an alibi because the property owned by his wife could mean he was in the area when Posch was killed, Lissl said.
"We are looking at the case from a third angle," he said of the new direction the investigation has taken.
Fritzl, 73, confessed Monday to imprisoning his daughter Elisabeth for 24 years in a warren of soundproofed cellar rooms, sexually abusing her, fathering seven children with her and discarding the body of an one, who died in infancy, in a furnace.
On Tuesday, tests confirmed that the retired electrician is the biological father of his daughter's six surviving children.
Three of the children were locked in the underground labyrinth with their mother for years.
Officials said a teenager conceived through incest is in critical condition at an Austrian hospital where she remains in a medically-induced coma. The 19-year-old is on dialysis and her condition has somewhat stabilized, the officials said. It was an appeal by doctors, mystified by her ailment, that brought the family's situation to light. One of those children died in infancy. Doctors said the other five are doing relatively well. One of the challenges they apparently face is getting used to daylight.
The case started unfolding on April 19 when one of the children held captive was found unconscious and was taken to a hospital. After receiving a tip, police picked up Elisabeth and her father on Saturday. Fritzl freed the captive children the same day.
Authorities say Fritzl led his wife to believe that Elisabeth had run away to join a religious cult when she disappeared.