Before the International Criminal Tribunal for the former Yugoslavia (ICTY), Grujic spoke about methods of unearthing mass graves and identifying victims from such graves in the area of Vukovar and eastern Croatia.
The Croatian official presented numerous documents on this matter.
Grujic said that 200 bodies had been exhumed from the Ovcara mass grave, of which 192 had been identified.
Those were wounded soldiers and civilians who had been transferred from the Vukovar hospital to the Ovcara farm when the eastern Croatian town fell into the hands of the JNA and Serb paramilitaries in late November 1991.
Grujic also spoke about the existence of the so-called secondary graves in which bodes were transferred from their primary graves. In this context he mentioned the Vukovar New Cemetery where bodies were reburied.
The ICTY has indicted the former JNA officers - Mile Mrksic, Veselin Sljivancanin and Miroslav Radic - for crimes against humanity, holding them responsible for the massacre of at least 264 Croatian soldiers and civilians taken from the Vukovar hospital to Ovcara.
Grujic will continue testifying on Friday.