Vladimir Vukcevic identified the witness as "Darko," who he said had observed medical procedures, Beta news agency reported Thursday.
The prosecutor described Darko as a "valuable source of relevant data." Vukcevic said the witness was under police protection.
Vukcevic is investigation allegations members of the Kosovo Liberation Army kidnapped Serbs and other civilians in Kosovo in 1999 and 2000, imprisoned them in northern Albania and sold their body parts on the international black market.
The prosecutor said the war crimes tribunal in Brussels had informed the proceedings would be conducted outside Pristina and Kosovo.