UN investigators say they have compiled secret lists of "4,000" perpetrators of serious crimes in Syria and hope the fall of Bashar al-Assad will help ensure accountability up to the highest level. "I think it's very important that the top-level perpetrators are brought to justice," says Linnea Arvidsson, who coordinates the work of the UN's Commission of Inquiry for Syria (COI), "rather than the lower-level perpetrators." IMAGES AND SOUNDBITES