Police are investigating claims that an Austrian man accused of imprisoning his daughter in a cellar for 24 years and fathering her seven children also raped one of his granddaughters, a U.K. newspaper reported Tuesday.

Cops believe Josef Fritzl, 73, may have kept his granddaughter Kerstin as a sex slave in the dungeon he built under his home in the village of Amstetten, the Daily Mail reported.

Police are waiting to interview the 19-year-old, who is in a medically induced coma after she emerged from the basement dungeon on April 19.

The news comes as a prosecutor announced plans to meet Fritzl on Wednesday, an official said.

• Click here to see photos of Josef Fritzl and the "House of Horrors."

The meeting between prosecutor Christiane Burkheiser and Fritzl is to take place in the prison in the Lower Austrian city of St. Poelten, where the man is being held, prosecution spokesman Gerhard Sedlacek said.

Sedlacek said it is unclear if Fritzl would agree to answer questions. Authorities say he confessed to locking up and raping his daughter Elisabeth and has since remained silent.

Fritzl has not been charged and remains in pretrial detention. His lawyer, Rudolf Mayer, has said he is preparing an insanity defense.

Chief investigator Franz Polzer said experts are continuing to sift through Fritzl's property for evidence, adding there may be more "hollow spaces" that need to be examined. It is unlikely investigators would find anything "dramatic" in such areas, he said.

Doctors, unable to find medical records for Kerstin, appealed on television for her mother to come forward. Fritzl then accompanied the mother, Elisabeth, to the hospital on April 26 and confessed to police.

Click here for more from the U.K.'s Daily Mail.

The Associated Press contributed to this report.