One of France's most high-profile political scientists and media figures, Olivier Duhamel, has admitted that he sexually abused his stepson decades ago, French media reports say.
The accusation first emerged in a book by his stepdaughter, Camille Kouchner, which was published in January.
She said her twin had been abused as a young teenager over two or three years.
Mr Duhamel cannot now be prosecuted as the statute of limitations for rape and sexual abuse has lapsed.
However, the 70 year old was questioned by police on Tuesday, in an inquiry opened by prosecutors to identify any other victims.
According to LCI news channel, Mr Duhamel admitted abusing his stepson as stated in Camille Kouchner's book La Familia Grande. He is not suspected of abusing anyone else, LCI adds.
Le Parisien newspaper said he told officers the abuse - which dates back to the late 1980s - had been an "unforgivable mistake".
After the claim emerged he resigned from all his positions, which included the chairmanship of the body that oversees the prestigious Sciences Po university, as well as all media roles.
He had presented a show on Europe 1 radio and was an analyst on LCI.
Camille Kouchner, 45, and her twin brother are the children of Bernard Kouchner - a former French foreign minister and co-founder of the Medecins Sans Frontieres (MSF) medical charity - and the academic Evelyne Pisier.
Mr Kouchner and Ms Pisier separated in the early 1980s and she went on to marry Mr Duhamel. Ms Pisier died in 2017.
In La Familia Grande, Camille Kouchner described the guilt she felt over not speaking out about the abuse - which her brother had until then wanted to keep secret.
"I was 14 years old and let it go ahead. I was 14, I knew and said nothing," she wrote.
She named her brother only as "Victor" to protect his privacy. Le Monde newspaper said that he was happy for his sister to speak on his behalf.
In the book, Camille Kouchner says that many of those in Mr Duhamel's and Ms Pisier's intellectual circle in Paris were aware of the abuse at the time.