Four years after the deadliest of the many mass murders that have scarred the Balkans this decade, the U.N. war crimes tribunal's forensic teams in their white suits and plastic gloves are still bent over decaying cadavers in humid tented enclosures near the Bosnian city of Srebrenica.

Assisted by scores of toxicologists, anthropologists, archaeologists and other specialists, the investigators comb the pits where blindfolded bodies were dumped after systematic executions. They search for clues that will establish how and on whose orders more than 7,000 Muslims perished at the hands of Bosnian Serbs over a few days in July 1995. If there is an end to their work, it is not clear when it will come.

Not far to the southeast, war crimes tribunal investigators have swarmed across Kosovo over the past two months. They have made almost daily discoveries of mass graves and other evidence of Serbian atrocities against Kosovo's ethnic Albanians during the 78-day NATO bombing campaign--raising the prospect that Yugoslav President Slobodan Milosevic, indicted with four associates for alleged war crimes in Kosovo, could stand trial in The Hague.

The quick, massive arrival of investigators in Kosovo suggests that the machinery of international justice may move toward a relatively swift resolution. Teams from the tribunal were preceded by demining missions, surrounded by guards and guns, accompanied by planeloads of volunteers from documentation services and forensic agencies of many nations and flanked by an incomparably more intense presence of U.N. personnel, aid workers and media.

But the dramatic events in Kosovo obscure the prevailing reality of prosecuting atrocities: The process is slow, fitful, frustrating, perhaps doomed to be forever incomplete. And the course of justice in neighboring Bosnia and Croatia offers daily reminders of how much time it can take.

Nearly four years after the end of the Bosnian conflict and seven years after the war in Croatia died down--conflicts that drove the United Nations to establish its first post-World War II war crimes tribunal--only half those known to be indicted are in custody, and only eight out of nearly 70 have been tried. Most suspects still at large are Serbs--Croatians, Bosnian Croats and Bosnian Muslims were indicted in smaller numbers--and among them are those with the greatest authority over the commission of atrocities.

As for the foremost political leaders of the era--Milosevic and President Franjo Tudjman of Croatia--they are, for now, unindicted for their roles in the Bosnian and Croatian conflicts and remain in power.

Will Kosovo be any different?

Louise Arbour, the tribunal's chief prosecutor since 1996, defends the pace of the its work in both theaters of war crimes as she prepares to leave for her native Canada and take a seat on its Supreme Court.

In Bosnia and Croatia, the International Criminal Tribunal for the Former Yugoslavia was feeling its way along a landscape pocked with the still-combustible remnants of bitter ethnic war and overseen by jittery foreign peacekeeping forces. By comparison, the Kosovo operation has been a rush job, according to Arbour, other prosecutors and knowledgeable specialists. Under pressure from the United States and other Western governments as the Kosovo crisis accelerated last year, the U.N. prosecutor's office, still taxed with a full schedule for the Bosnia and Croatia investigations, had to shift gears and reallocate staff and resources.

To prepare indictments during the spring, as the Kosovo war loomed and then exploded, the tribunal chose to single out six specific cases of Serbian massacres of Kosovo Albanians that they knew about from the accounts of refugees and other intelligence provided by NATO governments. That information formed the basis for the May 27 indictment of Milosevic and four top political and military leaders in the Yugoslav and Serbian chains of command. Serbia is the dominant republic of the Yugoslav federation, and Kosovo is a province of Serbia.

But within days of the bombing halt, as peacekeeping soldiers, humanitarian workers, journalists and war crimes investigators flooded into Kosovo, new evidence of hundreds of other massacre sites began turning up, some known to the tribunal, many not.

Those discoveries have prompted new pressures on the tribunal to deal with the proliferation of horrors, even though, according to Clint Williamson, a senior tribunal lawyer, "there's no way to do every crime scene, not even every major crime scene" in Kosovo. "You have to select events that are representative of what happened everywhere."

Its investigators, headed by some 20 team leaders from The Hague, are relying heavily on donated expertise from friendly governments and humanitarian organizations. They are operating under a waiver granted by the United Nations, the tribunal's ultimate paymaster, and no one knows whether that largess and those dispensations will last as long as the job requires.

Except for a spasm of anger from some NATO governments that the timing of the Kosovo war crimes indictments might upset delicate peace negotiations with Milosevic--a concern later shown to be unfounded--the tribunal's work on Kosovo in general has been a useful instrument to Western powers seeking to demonize the Serbian leader in the eyes of the world and his own people.

In Bosnia, by contrast, the tribunal's attempts to pressure recalcitrant NATO governments to arrest leading Bosnian Serb suspects have seldom been welcome, and were executed slowly and selectively. The 1995 Dayton peace accords, which ended the Bosnian war, authorized NATO forces to detain war crimes suspects but did not require them to do so. So it is that Radovan Karadzic, the former Bosnian Serb president, and Ratko Mladic, the top Bosnian Serb general during the war, remain free despite their indictments.

"With an ambiguous mandate, it's always easier to back away from an arrest, to be sure you don't ruffle any feathers. They always tell us, 'It's not the right time to do it.' It's never the right time to do it," said Williamson, who has worked on tribunal prosecutions in Croatia, Bosnia and Kosovo. "But in Kosovo we have had a commitment from the start at the highest level in theater, both politically and militarily, to cooperate with us. So people down the chain of command don't have as much room to maneuver out of it."