(CNN) -- Thousands of mourners and supporters crowded hillsides near Srebrenica, Bosnia-Herzegovina, Sunday, to mark the 15th anniversary of the massacre of nearly 8,000 men and boys.
More than 50,000 people witnessed the burial of 775 newly identified genocide victims at the Centre Potocari -- the official Srebrenica Genocide Memorial where 5,000 genocide victims are buried -- a few miles outside the city, according to Bosnia-Herzegovina's official news agency, Fena.
In 1995, Potocari served as a safe-haven for Bosnian Muslims. They were under the protection of a U.N.-Dutch-led mission.
The massacre of men and boys occurred when ethnic Serb troops overran the United Nations safe area.
The five-day slaughter was the worst European massacre since World War II and was described by the U.N. War Crimes Tribunal as "the triumph of evil." Witnesses, human rights investigators and court testimonies documented the summary executions, as well the rape and murder of women.
Sunday's burial was attended by top international officials, including a U.S. delegation whose members read a statement from President Barack Obama. In the statement, Obama called the Srebrenica massacre an "unimaginable" tragedy that has left an indelible "stain on our collective conscience."
"Fifteen years ago today, despite decades of pledges of 'never again,' 8,000 men and boys were murdered in these fields and hills," said in the statement.
"They were brothers, sons, husbands, and fathers, and they all became victims of genocide."
Obama also called on all governments to "redouble their efforts" to find and prosecute those responsible for the tragedy.
"This includes Ratko Mladic, who presided over the killings and remains at large," he said, referring to the former Bosnian Serb army commander.
Mladic has been charged with genocide and remains a fugitive. Bosnian Serb president Radovan Karadzic is now on trial at the tribunal in The Hague.
Fena news agency reported that Boris Tadic, president of Serbia, also attended the burial memorial of the genocide victims and promised to find Serb war criminals.
"I, as the president of Serbia, will never give up finding them, and especially Ratko Mladic. When that ends, I will consider a part of my job completed," Tadic said.
"But before that, it is important that people reach out to one another and to continue living as people. A lot of time has passed, and I have done everything possible for Mladic to be arrested," he said.