Almost every evening, reports filter into NATO's Kosovo headquarters about new discoveries of mass graves filled with bodies of ethnic Albanians killed by Yugoslav and Serbian security forces.

The reports, which have come from NATO troops, villagers and relief workers as they make their way systematically through Kosovo's battle-scarred landscape, are providing a fuller picture of the scope of mass killings disclosed in general terms by ethnic Albanian refugees during the 11-week war.

Already, the Western alliance has collected more than 390 reports of what it formally calls "mass burial sites," or sets of corpses--an average of more than 10 reports a day since the war ended on June 10. Sixty-nine reports have been confirmed by the U.N. war crimes tribunal, with the number of corpses involved in each of these said to range from a few to more than 100.

Western investigators say that if all the reports collected by NATO are eventually confirmed, the tally in human lives could be at least 5,500. But no official here expects the count to stop there, and many say it likely will climb to more than 10,000 as returning refugees and NATO troops venture farther into remote areas.

That judgment is borne out in part by the work of the Council for the Defense of Human Rights in Kosovo, an ethnic Albanian organization that is collecting names from each village of all residents killed in the war. An organization official said that a list of more than 4,000 names have already been compiled.

"It is generally accepted that the number of bodies is more than 4,900," said Lt. Col. Robin Clifford, a spokesman for Lt. Gen. Mike Jackson, the British commander of NATO peacekeeping forces. "It is reasonable to assume it will be double that. There are multiple body sites still being found."

Clifford added, however, that he does not think "we are ever going to know" the real death toll. Some corpses were hidden, some were burned or removed from Kosovo, and some have been claimed by relatives from identifiable grave sites since the war.

Louise Arbour, the chief prosecutor for the Hague-based tribunal, said "It is premature for the tribunal to lend credibility to any figure [of the number of dead] at this stage. It is not outside the realm of possibility that the figure [of 10,000] is in that range, but at this point, with the methods we're accustomed to using, we just don't know."

The NATO summaries of mass grave reports are written in military style, but nonetheless provide a glimpse of what residents and foreigners alike are encountering: A charred torso was found in the hall of a burned house, one report says. Two decapitated bodies were found in a ditch, said another. Others say some bodies were found in wells. A tractor cultivating a field pulled up body parts. There were 158 bodies confirmed in a mass grave, with four more in a nearby house and stream. A grave site was found with 51 people reportedly killed by a single Serbian gunman.

Western officials here say the war in Kosovo followed a pattern set during the 1992-1995 Bosnian war, in which an estimated 200,000 people, mainly civilians, were killed. Those confirmed to have been summarily executed in Kosovo include infants, girls, mothers, the handicapped, grandmothers and grandfathers--even some in their eighties and nineties.

But war crimes investigators and local officials say middle-aged men and teenage boys make up the largest group of victims. Witnesses in many villages have said these two groups were separated from women and small children and ushered away to be shot. Arbour said there were "conglomerations of males of military age" at the grave sites.

Several thousand men were apparently slain on suspicion that they already were, or might one day become, members or supporters of the Kosovo Liberation Army, a rebel organization that fought for 16 months for Kosovo's independence from Serbia, Yugoslavia's dominant republic. But officials here say Serb-led security forces killed a great many boys in an attempt to eradicate a new generation of separatist rebels.

Included in the group of victims were intellectuals, the wealthy and some local political figures, villagers said, but it is hard to assess whether each group was targeted deliberately. Western officials say it appears that some effort was made to target ethnic Albanians who had worked closely with Western organizations in Kosovo, including the Organization for Security and Cooperation in Europe, which monitored Yugoslav military forces before the war.

Several high-ranking OSCE officials have said that some Serbs hired by the group were later found to be agents of the Belgrade government who obtained a copy of its personnel and contract lists and that the information was later used to hunt down OSCE employees and target property owners who had rented space to the OSCE.

While most wartime accounts of massacres were provided without physical evidence and were denounced in Belgrade, virtually anyone who drives into a Kosovo village now can readily find someone to show him mounds of freshly turned earth and handwritten lists of victims.

Investigators say there is ample evidence that in the middle of the war--after the allies began to publicize war crimes allegations--Belgrade government forces tried to conceal what they had done. Some mass graves were excavated with bulldozers and the contents carted away in trucks, they say. Some remains may have been burned, while others were reburied elsewhere.

The task of specifying the number and identities of those killed has been complicated by the massive internal movements of people during the war--involving as many as 700,000 according to NATO satellite imagery. Some victims are known to have been killed far from home along with other refugees who cannot be identified.

Moreover, determining how many bodies are present at each site requires painstaking work. The tribunal suspects there are 2,150 bodies at the 69 confirmed mass burial sites. So far, tribunal investigators have found the remains of 869 people.

One of the largest surprises for war crimes investigators has been the amount of evidence left behind. Bone fragments have been found even at grave sites dug up by government troops so the bodies could be moved. Some villagers who stayed near their homes or hid in the hills kept meticulous diaries of what they saw, and in some cases made videotapes of mass grave sites. Residents of virtually every village are identifying the missing and recording accounts of atrocities.

The Kosovo human rights council estimates that more than 3,000 people are missing. As a result, many Kosovo residents say they have pinned their hopes of seeing their relatives again on reports that the Belgrade government is holding nearly 2,000 ethnic Albanians prisoners, including hundreds bused out of Kosovo in the final days of the war.

But the prisoner lists Belgrade gave to the Red Cross a week ago reportedly contain duplicated names as well as names of ethnic Albanians arrested before the war. So the number of missing people who may still be alive is unlikely to be as large as relatives hope.

Correspondent Charles Trueheart in Dubrovnik, Croatia, contributed to this report.