U.N. war crimes investigators have begun questioning a former Croat general about the mass killing of Serb civilians in two military campaigns during the 1990s.

General Mirko Norac is being interrogated about his role in a 1993 military operation near the Adriatic coastal city of Gospic and a 1995 offensive that drove Serb forces from Croatia. The questioning, which began Monday at a prison on the outskirts of Zagreb, is expected to last several days.

A Croatian court sentenced General Norac to a 12-year prison term in March for the deaths of about 50 Serb civilians in the Gospic operation.

General Norac, who is considered a hero by nationalists in Croatia, was the highest-ranking officer to be convicted of war crimes by a Croatian court.