Relatives of thousands of Bosnian Muslims killed in Europe’s worst post-war atrocity began a court case yesterday to sue the United Nations over their deaths.

More than 6,000 relatives of the victims of the 1995 Srebrenica massacre are taking part in the case, which could set a legal precedent for U.N. responsibility in conflict zones.

Lawyers for the families argue that the U.N. was culpable for failing to stop the execution of more than 8,000 men and boys in what had been designated a U.N. “safe haven” during the Bosnian war.

A lightly-armed contingent of Dutch U.N. peacekeepers failed to prevent the mass killings. Some even helped to separate men and women before the killings started.

The Bosnian Serb leaders who are accused of genocide for allegedly ordering the massacre, Radovan Karadzic and Ratko Mladic, remain on the run.

The victims’ relatives want the U.N.’s immunity from prosecution waived so they can sue for compensation, with damages estimated at $4 billion.

The president of the Mothers of Srebrenica association, Munira Subasic, said, “The genocide happened here under the protection of the United Nations, before the eyes of the United Nations.”

Judges in the case, which is taking place at the Hague, are likely to take several months to rule on U.N. immunity.