You could smell the mass grave at Cerska long before you could see it.

The sickly, sweet smell of the bodies came wafting through the trees lining the dirt track up to the grave. The killers had chosen their spot well, an obscure rise off a rutted road few needed to travel.

Investigators with the International Criminal Tribunal for the Former Yugoslavia (ICTY) had discovered the grave. And the stench that hovered in the air indicated they were exhuming it, collecting evidence for war crimes cases.

The corpses were dressed in civilian clothes. They had gunshot wounds to the back of their heads. Their decaying hands were bound behind their back. These men and boys, forensic experts at the scene said, had been gunned down in cold blood.

The Cerska grave is one of several exhumed in Bosnia that help explain the fate of approximately seven thousand Bosnian Muslim men and boys from Srebrenica, who disappeared after Bosnian Serb forces overran the UN safe area in July 1995. Bosnian Serb leaders asserted that Srebrenica's men, wielding arms, were killed in combat.

The grave proved otherwise.

Individual and mass graves provide vital evidence to war crimes prosecutions, especially those involving extrajudicial executions and targeting of civilians. Forensic experts over the last twenty years have worked to exhume and examine graves in Argentina, Guatemala, El Salvador, Honduras, Ethiopia, Mexico, and Iraqi Kurdistan. Exhumations in Argentina, for example, helped show that many of the thousands of civilians who disappeared during the juntas had been executed; that forensic evidence was presented during the 1985 trial of nine Argentine generals, five of whom were later convicted.

In recent years, forensic teams have exhumed mass graves in Rwanda and the former Yugoslavia, some of the largest graves yet discovered. Evidence from the exhumations will be a key part of upcoming war crimes cases. For example, evidence from graves like Cerska, combined with witness testimony, would be part of the case against former Bosnian Serb leader Radovan Karadzic and army commander Gen. Ratko Mladic. Both men have been charged with war crimes, genocide, and crimes against humanity.

To prove genocide or crimes against humanity in the case of Srebrenica, prosecutors would have to show that Bosnian Muslims were deliberately targeted for mass executions. Forensic evidence will help establish that the dead in a mass grave are Bosnian Muslim civilians and that they were executed.

Mass graves themselves can be a violation of international law. The Third and Fourth Geneva Conventions and Additional Protocol I contain provisions governing the proper burial, identification, and registration of those killed in war. Prisoners of war, for example, must be "honorably buried" in graves that bear information about them.

But the right to exhume a mass grave or to halt tampering with a grave is not clear under international law. UN General Assembly resolution 3074, adopted in 1973, calls for States to cooperate in war crimes investigations. Articles 32 and 33 of Additional Protocol I require parties to search for missing persons after hostilities end, and otherwise assist in finding out their fate. But an individual country does not have to allow suspected mass graves to be examined.

Not all mass graves contain victims of war crimes or atrocities. Some may hold the bodies of hurriedly buried combatants. Witnesses and survivors will help identify which grave is which. Even then, the mass grave may not be obvious.

Experts often comb through a field or forest to find a mass grave. They will look for abrupt changes in vegetation to indicate recent burial activity, or changes in the texture and color of earth. Depressions or mounds are another sign digging has recently taken place.

Reporters who come across what they believe to be a mass grave should not interfere with it. Mass graves are often mined or strewn with unexploded ordinance. Disturbing a grave might also compromise the evidence it contains. Do not try to excavate, or collect anything protruding from the grave. Only photograph the grave and mark its location on a map.

Mass graves can be easily tampered with and the evidence they contain lost forever. It is important to exercise judgment about whom is notified of a suspected mass grave. Two starting points are the International Committee of the Red Cross (ICRC) and the UN High Commissioner for Human Rights. The Boston-based Physicians for Human Rights, which sends forensic teams to examine graves around the world, is another. Once forensic experts arrive on the scene, they will conduct their search with an archaeologist's precision. Each part of the human skeleton-some two hundred bones and thirty-two teeth-has its tale to tell.

A forensic team will begin by probing the grave, often with a metal rod, seeking to test its consistency and detect the smell of dead bodies. Once investigators have dug down to the level of bodies, they will sift the earth for shreds of evidence and dust off each body. Bodies are carefully examined before being removed. Valuable evidence can include blindfolds, bullets, and bonds that will indicate how a victim was killed. Jewelry or papers help with identification.

The science of determining the cause of death is complex. An expert examining a bullet wound can determine where a person was shot, the range of the shooting, and the angle at which the bullet entered-all clues to whether someone was executed or not. Identification is the next step. Experts rely on witnesses, who may know who is in which grave. Accurate dental records help make a match between a body and a missing person. DNA testing can also be used to help identify victims.

Success varies. In Rwanda, identification is almost impossible, due to a lack of records and the vast size of graves. But experts are optimistic they will be able to identify most of the two hundred bodies exhumed from Ovcara, Croatia, thanks to a list of who was in the grave. They are less optimistic about the Cerska grave, given how many people have gone missing.

But grieving mothers and wives still hope they will succeed. "Bring his body to me," said Hatidza Hren, a Bosnian Muslim searching for her husband. "I will recognize his bones."

(See disappearances; medico-legal investigations)

Elizabeth Neuffer is a reporter and former European bureau chief for the Boston Globe. She is currently working on a book on post-war justice issues in Bosnia and Rwanda.