The first protected witness was interrogated by the prosecution and the defence behind closed doors.
Spokesman Bruno Vekaric told reporters the Prosecution for War Crimes was satisfied with the course of the trial against 17 members of Serb troops accused of killing 192 Croatian prisoners of war at Ovcara farm in eastern Croatia in November 1991.
All defendants, indicted on 4 December 2003, pleaded not guilty. The trial began on March 9 this year.
Belgrade attorney Nikola Barovic, who represents the victims' families, told Beta news agency he was satisfied with the course of the trial, and that the first insider witness confirmed the allegations in the indictment against some of the accused.