The grave stretches over 5,000 square meters (53,820 sq. feet) and is 10 meters (about 30 feet) deep.
For Meron, a Holocaust survivor from Kalisz, Poland, the visit was a painful experience.
"This place has a very, very special resonance to me personally because it looks little bit like the place in Kalisz ... where my mother was killed. So it means to me more than just international law," he said.
"It is very difficult for me to speak at this place, where one stands face to face with the horror a man can do to another man," he said.
Authorities hope that many of the 1,200 still missing people from the area will be found in Tomasica.
The area of Prijedor was a site of severe crimes against humanity committed by Christian Orthodox Serbs against Muslim Bosniaks and Catholic Croats.