WASHINGTON -- As Bosnian Serb forces closed in on the United Nations "safe area'' of Srebrenica on July 11, some 12,000 draft-age Moslem men terrified of being killed by the advancing Serbs gambled their lives on an arduous 100-kilometer trek through enemy-held territory to reach the Moslem stronghold of Tuzla.


Significantly less than half of the mainly unarmed men made it to safety, according to information collected by international human-rights organizations and Bosnian officials. Nearly all of the remainder were butchered by troops under the command of General Ratko Mladic, the Bosnian Serb military commander, despite repeated promises he gave the refugees that he would personally ensure their safety.


The accounts now available indicate that the massacres in the Srebrenica area were the worst atrocities committed in Europe since World War II.


"Being there, and seeing so many people executed was terrible," said Hurem Suljic, a 55-year-old Moslem who survived a massacre near the town of Karakaj by staying motionless beneath a pile of dead bodies. "Anybody who moved or screamed was killed ... I was afraid someone could be alive on my back, and if he moved, they would shoot us again. Fortunately, they were all dead."


Although reports of mass killings began to circulate soon after the fall of Srebrenica, the full horror of what took place became apparent only much later as survivors of the long march told their stories to journalists, diplomats and human-rights activists. A detailed reconstruction of events by The Washington Post now suggests that there were at least five or six separate massacre sites, where large numbers of Moslem men were buried in mass graves, as well as dozens of other places where individual killings took place.


The following description of what befell the inhabitants of Srebrenica was put together from accounts given by survivors and witnesses in interviews conducted in the Moslem-controlled Tuzla region in recent days. It also draws on testimony collected by human-rights organizations including Human Rights Watch, Amnesty International and the UN Human Rights Commission.


Serb fighters, many of them dressed in black uniforms with white T-shirts, decimated the Moslems with mortar and heavy machine-gun fire. Men coming up from the rear of the column came across piles of bodies dumped in streams. Many had slit throats. Others were decapitated.


Some men committed suicide rather than give themselves up. The UN commissioner for human rights, Tadeusz Mazowiecki, reported that one man shot himself in the face, but failed to kill himself. He had to plead with friends to finish the job. A marcher, Ramiz Becirevic, described an incident in which a man took his own life with a grenade and killed five other people as well.


Thousands of men were rounded up by Serb forces as they attempted to cross the main roads at Kravica and Nova Kasaba. Some were tricked into surrendering by Serb soldiers driving captured UN vehicles and masquerading as UN troops.


The best-documented incidents of mass killings occurred in the Nova Kasaba area. On July 12 and 13, U.S. satellites and spy planes took photographs of hundreds of people in a soccer stadium, later identified by eyewitnesses as one of several Serb detention camps. Several days later, American aerial reconnaissance recorded an empty stadium, together with four large patches of freshly dug earth in nearby fields and new truck tracks, leading to U.S. charges at the UN that the Bosnian Serbs killed the men in the stadium.


According to numerous eyewitnesses, the operation to round up draft-age Moslem men was personally supervised by Mladic, who was indicted by the International War Crimes Tribunal in The Hague in July for his alleged participation in earlier atrocities.


On July 13, Mladic's men transported 2,500 prisoners to Karakaj where the men were blindfolded and trucked to an execution site. According to testimony collected by Human Rights Watch, the prisoners were machine-gunned after being made to jump out of the trucks.


The executioners moved among the corpses, looking for any sign of life and occasionally putting an extra bullet through someone's head recalled Avdic, a 17-year-old Moslem.


There were moans of "kill me, just finish me off."