Reconstructing 50-year-old personnel records for nearly 4,000 soldiers, unearthing yellowed intelligence reports and using computers to enhance fading aerial photographs, the Army has spent the past 16 months engaged in an extraordinary enterprise: trying to disprove allegations that 1,200 black soldiers were massacred during a racial disturbance at an Army camp in Mississippi during World War II.

No hard evidence of the massacre, first publicly alleged last year in a self-published book by a Mississippi banker, has ever surfaced. Now, through an unprecedented reconstruction of personnel records, the Army has accounted for every man who served in the unit that allegedly suffered the atrocity, the 364th Infantry Regiment, while it was posted at Camp Van Dorn in southern Mississippi in 1943.

And yet, the massive official effort to refute the allegations has failed. This week, the NAACP, which had originally pressed the Army for an explanation, quietly asked Attorney General Janet Reno to conduct an independent investigation of the alleged killings and the cover-up that would have been necessary to keep them from being reported for 50 years.

"We just feel that these charges are too serious for us to say, 'Great. The Army's correct. Let's move on,' " said John White, an NAACP spokesman.

The continuing controversy over the alleged Van Dorn killings threatens to harm the Army's long-standing effort to build a reputation as an institution where African Americans and other minorities can flourish. And it offers a case study in how the legacy of segregation can make it difficult for an agency such as the Pentagon to dispel damaging allegations about past activities.

The Army's history of racial discrimination feeds the problem, say outside experts. "If you believe the Army report on [Van Dorn], that is one down and you have all these others to go," said John Hope Franklin, the historian who headed President Clinton's advisory commission on race. "It doesn't speak to the question of the general mistreatment of African Americans during the war. That there was widespread mistreatment, that is a given."

Even though the NAACP has no independent basis for believing that a mass killing did take place, White pointed to other reports of mistreatment of black troops that are well-documented. "If the Justice Department says what the Army found to be true is true, that's the end of it," White said. "But we think that perhaps the Army owes an apology to some of the soldiers who were victims of racism down there."

The 364th suffered its share of mistreatment, according to the Army's official history of World War II and the report rebutting the charges of a mass killing, which was provided to The Washington Post in advance of its public release Thursday. The record includes inadequate housing, training and recreation facilities and, more seriously, two shootings of soldiers, one by a small-town Mississippi sheriff and the other by military police, that prompted riots by black soldiers.

"In 1943, America was not exactly a great place for black people and this institution was guilty of its own failings, but nonetheless the allegation of the slaughter is not true," said William E. Leftwich, III, deputy assistant secretary of defense of equal opportunity.

The story of the alleged Van Dorn massacre first surfaced last year in "The Slaughter: An American Atrocity" by Carroll Case, who has worked as a banker, artist and writer in McComb, Miss. Case, 59, who is white, wrote that as he grew up in southern Mississippi he heard "hushed rumors" of a mass killing of black troops. Then, in 1985, a maintenance man at the bank where he was president recounted his service at Van Dorn.

In addition to some of the well-known altercations involving the 364th, the janitor, William Martzell, who has since died, described a night in the fall of 1943 when white troops and military police armed with machine guns surrounded the 364th's barracks. As quoted by Case, Martzell said, "We had the whole area sealed off--it was like shooting fish in a barrel. We opened fire on everything that moved, shot into the barracks, shot them out of trees, where some of them were climbing, trying to hide. . . ."

Case claims 1,227 soldiers were killed that night, about a third of the regiment. Their bodies, he says, were loaded into rail cars and buried on the outskirts of the camp in trenches subsequently covered by a 12-acre lake. The Army report includes a series of aerial photographs of the camp and surrounding area taken since 1943 that show no such lake or any other sign of a mass grave.

Martzell's service record shows that he had not been posted at Van Dorn at the time of some of the events he purported to describe first-hand. Case has no clear explanation of why the shootings took place, no firm date for the event nor the names of any of the other participants or victims. Although Case presents accounts from at least two other local residents who claim to have witnessed the killings, in 15 years he has never encountered a member of the 364th or any of the other 30,000 soldiers at the camp at the time who remember the alleged atrocity.

The Army thus far has contacted about 20 surviving members of the 364th who deny that a massacre took place, and is searching for more.

"It was completely impossible for this to have occurred in my unit without me knowing about it, completely impossible," said Richard E. Douglas, who was a private working in the headquarters battalion of the 364th at the time.

Another surviving soldier, Louis Cooper, said he had no knowledge of any mass killings, and that if anything like that had happened, family members would have contacted the black newspapers in northern cities, which were carefully tracking the treatment of black soldiers. "It just would not have escaped notice because everything that was happening at the time was getting written up," he said.

While Case, the NAACP and the Army have found accounts of other events involving the 364th, none has appeared hinting at the alleged massacre.

"There is a great deal of material from the time recounting the condition of black soldiers in the Army in 1943, including letters home from the soldiers, letters by family members sending complaints to newspapers, Congress and the NAACP itself, reports from Army investigations, internal Army communications, newspaper accounts and much more," said Lt. Col. Charles Graul of the Center for Military History, who led the Army investigation.

"The fact that this alleged slaughter does not appear anywhere in that documentary record is strong proof that this simply did not happen," he said.

Case argues that the lack of any accounts by members of the unit shows that those the Army wanted killed were separated from those to be spared. As to the Army's reconstruction of personnel records for the more than 4,000 soldiers who served with 364th at Van Dorn, Case said, "I believe the records have been falsified."

So far the NAACP is not willing to allege that the Army is lying about what the records show, but after its own internal investigation, an inspection trip to Mississippi and months of internal debate, the civil rights group is also reluctant to put the allegations of the slaughter to rest. And, according to some experts, that may be the most significant aspect of the whole affair.

"This does not tell us anything about the actual history of blacks in America because there is no proof that it happened, but it does reveal something very interesting about the way people see that history," said John Sibley Butler, a professor of sociology and management at the University of Texas at Austin.

"So many bad things happened to black soldiers during that time period that something like this supposed slaughter could have happened, and because of that, people can put aside the question of whether or not there is evidence and simply believe that it did happen," Butler said.

CAPTION: The 364th Infantry Regiment marches in review on Attu Island on VJ Day in 1945.