Tripoli - The ICC issued an arrest warrant for Saif al-Islam last year after prosecutors accused him and others of involvement in the killing of protesters during the revolt that eventually toppled his father, who ruled with an iron fist for 42 years.
It has called on Libya to hand Saif al-Islam over but Tripoli has insisted he will be tried in his home country after his capture in the southern desert in November.
ICC chief prosecutor Luis Moreno-Ocampo travelled to Libya this week as part of ICC investigations into crimes committed during Libya's war, meeting the head of the ruling National Transitional Council and chief prosecutor.
"I understand he has more than 30 witnesses, he's got documents, he has interceptions, so I understand he has a strong case but I don't know the details," Moreno-Ocampo told a news conference on Saturday, adding he had not seen the information as it remains confidential at this stage of the investigation.
Tripoli is to outline its case to the ICC on April 30 as to why it should put Saif-al Islam on trial.
Libya has the right to try him on its soil following his arrest there. The ICC will only act if a country is deemed unable or unwilling to investigate or prosecute, for instance when its legal system has collapsed.