Did X1 witness, compelled to murder, Katrien De Cuyper at Castle Kattenhof in 1991? We won't know for sure unless the investigation is reopened with the knowledge of X1's testimony is still credible even though minor details have differed. With all the connections between the people accused by X1 are put into view, it enhances the case rather than discredit it.
X1 apparently witnessed Carine "Clo" Dellaert's murder, at which she saw similar people involved in murders of other girls. The life of Carine is briefly examined and X1's testimony is determined to have been credible.
"I met Nihoul in a restaurant in Brussels. 'I am the Monster of Belgium,' he roared at me by way of greeting. He is confident he will never come to trial and that the evidence against him will never be heard by any jury. During the course of our meal he, apparently playfully, grabbed me, tickling, and finally pulled me over on to him in the restaurant booth until I had to appeal to my colleagues for rescue." (May 5, 2002, Olenka Frenkiel for The Guardian, 'Belgium's silent heart of darkness')
All roads in the Dutroux affair, lead to Jean-Michel Nihoul.