From a huge archive of documents about the Vietnam War declassified in recent months, new evidence is emerging that some American pilots held prisoner in Laos were not released at the end of the war, and that U.S. intelligence officials might have known where some of them were.

The Defense Department lists 330 Americans, almost all pilots and crew, as missing in action in Laos. Most were certainly killed when their planes crashed in the remote jungles of the mountainous, sparsely populated country.Officially, only two American fliers, Col. Charles Shelton and Lt. Col. David Hrdlicka, are known for certain to have been alive in custody of pro-communist Pathet Lao rebels. Shelton and Hrdlicka died in captivity in the 1960s, Pentagon officials believe. No other reports, whether from human sources or aerial photographs, of Americans held prisoner by the Pathet Lao have ever been verified, according to the Defense Department.

But declassified documents from the State Department, the Central Intelligence Agency and the Defense Intelligence Agency provide some support for those who argue that the number of prisoners was considerably higher, perhaps as high as 41 Americans.

Some military intelligence specialists and prisoner-of-war activists have believed for years that U.S. prisoners may have been left behind in Laos. Senior officials of the Nixon administration, in anguished testimony before a Senate committee in September 1992, acknowledged that they feared it was true at the time but said they decided then there was little they could do.

The truth about Laos has eluded military specialists and diplomats for two decades, and Laos remains the black hole of the long, bitter story of the more than 2,200 American service personnel still unaccounted for from the nation's longest war.

Of the 591 Americans released by North Vietnam in "Operation Homecoming" in 1973, only nine had been captured in Laos, and those nine were in custody of the North Vietnamese, not the Laotians. None had been held by the Pathet Lao in areas of Northeast Laos where, according to some intelligence documents, groups of downed U.S. fliers were kept prisoner. Aside from Shelton and Hrdlicka, the identities of such fliers taken prisoner, if in fact that occurred, and their ultimate fates remain unknown.

In the negotiations with North Vietnam that produced the Paris Peace Agreement and ended U.S. involvement in the war in January 1973, President Richard Nixon's national security adviser, Henry Kissinger, repeatedly sought assurances from the North Vietnamese that they would deliver all U.S. prisoners "throughout Indochina" in the postwar prisoner exchanges.

The United States never acknowledged officially participating in a war in Laos, and Laos was not a party to the Paris accord. U.S. negotiators believed, however, that the Pathet Lao communists were, in Kissinger's term, "stooges" of the North Vietnamese, and would deliver their prisoners if ordered to by Hanoi.

U.S. officials were shocked when only nine were delivered from Laos, according to declassified documents and testimony at the 1992 hearings.

Lawrence Eagleburger, then a senior Pentagon official and later secretary of state in the final months of the Bush administration, wrote in a memo to his then-boss, Defense Secretary Elliot Richardson, that after the last of the acknowledged prisoners had been released the United States should stage a "demarche," or diplomatic initiative, on the Laotians about the rest.

"This initiative should plainly and forcefully assert that the U.S. will no longer play games with the POW issue in Laos," said the memo, written a week before the final prisoner release. The Laotian communists "should be told that we have reason to believe they hold additional U.S. prisoners, and we demand their immediate release, as well as an accounting and information on all those who may have died."

But the United States had little leverage over Laos or North Vietnam. Kissinger, furious at being accused, in effect, at the 1992 hearings of having knowingly abandoned U.S. prisoners, argued that Congress would not have permitted a resumption of the air war in a campaign to force the release of prisoners whose location and identities were unknown, if indeed such prisoners existed.

Nixon, in an address to the nation at the conclusion of Operation Homecoming, said, "All of our American POWs are on their way home." Later in the same speech, he said provisions of the Paris agreement regarding Laos "have not been complied with," but he did not indicate there might still be U.S. prisoners there. Several times in the next few months of 1973, he repeated that all prisoners had come home.

But the declassified documents show there was intelligence information that the Pathet Lao held some U.S. fliers in caves near Pathet Lao headquarters in Sam Neua, in northeastern Laos, near the border with Vietnam.

Asked by a House committee in 1976 how it could be that none of more than 300 Americans lost in Laos could be a prisoner, Vernon Walters, then deputy director of the CIA, replied in writing that "this question has been very disconcerting to the intelligence community also. We have information that some of these 300 individuals survived their shootdown incident. Admittedly, the number is small."

If any of the intelligence information was correct, the apparently inescapable conclusion is that some men were abandoned to their fates when the last U.S. troops left Indochina, unless the Pathet Lao killed them, as some U.S. officials believe.