 From 1995 he worked for two years as anti-mafia prosecutor in Caltanissetta (Sicily) at the anti-mafia Directorate.From 1998 to 1999, Kessler was Deputy Head of the OSCE Kosovo Verification Mission in Pristina, in charge of Police and Justice matters.As judicial expert, he also participated to several evaluation missions of the European Commission and Council of Europe in Eastern Europe.