On May 2, 1975, the fleeting shadow of a sampan slipped through the marshy Vietnamese waterways: It scooted up one of the myriad tributaries past the village of Xeo Ro. Huddled in the back of the boat were two Americans-a gaunt white man a huskier black man - with their hands lashed behind their backs. Curious villagers hurried to a dock to stare.

Five months later, two other emaciated Americans were seen in the same vicinity. Both were white, both "very thin." Reported a witness: "They lay in a motorized sampan. There heads were shaven like monks'. Their wrists were tied with ropes behind their backs."

Similar haunting reports - here, two scraggly Americans begging for cigarettes, there, a scrawny white man with a faraway stare - have been brought to us. The stories can't be verified, but neither can they be disproved. They linger as an ugly memory of a war America would like to forget.

The fate of 697 Americans, missing in action, still nags the Pentagon, which is trying to close the books on theVietnam War. Many of those men were definitely alive in communist prison camps. They were seen by comrades or spotted in photographs. But where are they now? No one has explained what happened to them.

To find out, President Carter established the Woodcock commission in February 1977. Its members traveled to Vietnam to take up the case histories of the MIAs with the Vietnamese. After a painstaking review, the commission concluded: "For reasons of terrain, climate, circumstances and passage of time, it is probable that no accounting will ever be possible for most of the Americans lost."

A House select committee conducted its own separate investigation of the missing Americans. Chairman G.V. Montgomery (D-Miss.) told us sadly: "I wish it weren't true, but I know no Americans are still alive."

Yet whispered reports continue to drift out of Vietnam. They are brought out by refugees who furnish tantalizing fragments of information that often seem authentic. Three of the 10 members of the House Committee refused to accept the conclusion that no Americans would be found. Even Montgomery admitted to us: "It is conceivable we might make one or two mistakes. But this can't go on forever."

An unofficial delegate, who toured Indochina with the Woodcock commission, also disagreed with its findings. He is Dr. Roger Shields, the deputy assistant defense secretary in charge of MIA affairs from 1973 to 1976. He wrote a scathing letter to Defense Secretary Harold Brown, informing him, in effect, that "the Vietnamese sold us a bill of goods. I did not believe what I was told."

The State Department's MIA expert, Frank Sieverts, had dismissed most refugee sighting as inaccurate. Yet he acknowledged: "Of course, there is the possibility of collusion by Vietnam. We are ultimately at their mercy."

We have tried in vain to verify the refugee reports. We have carefully extracted details from letters that the refugees have written. Our reporter Josh Levin has questioned a dozen refugees directly. A Vietnamese woman in Cheverly, Md., placed an ad in a Vietnamese-language newspaper. She has received 12 responses, with details about MIAs. Some offered to provide more information if their identities and security could be protected.

But Sieverts is still skeptical. "Refugees often want to make a good impression us," he said: "We can't deal with imagination in a subject as important as this."

The Defense Intelligence Agency has analyzed some of the refugee reports. It found the story of the two Americans with shaven heads "quite doubtful" and another account suspect because of the witness's "obvious desire to call attention to [a] request to be allowed entry into the United States."

But the DIA cancelled an interview with Trinh Hung, now living in Philadelphia, who told about seeing the shorn Americans. His written account was rejected because it mentioned that the pair were on their way into the U Minh forest. The DIA claimed that American prisoners in South Vietnam "were transferred north for detention," so Hung's story was "contrary to known events."

But we have received other accounts of prisoners being led into U Minh forest. According to one report, 15 cinderblock prison cells were located in this forest in the spring of 1975. Fifteen American soldiers, "probably officers," five black and 10 whites, were reportedly held there.

Nor do all the reports we have seen, appear contrived to impress American authorities. Many of them in fact, appear hauntingly legitimate. We have also interviewed several refugees, who contrary to official statements, were never questioned about MIAs by American officials.

It almost seems as if the Pentagon is overeager to close the files on the MIAs. That was recommended, in fact, by the House select committee. The Pentagon's four service secretaries subsequently mulled over the folders of 286 MIAs. All of those reviewed have been reclassified "presume dead."

The military brass may be correct to believe there is no hope of recovering any more MIAs alive. Certainly, the families of MIAs shouldn't have their hopes raised unwarrantedly.

But the Pentagon should not close the files on those men prematurely. There are still reports that haven't been analyzed. The generals, who were so willing to send young men to fight in Vietnam, owe it to them to wait until the last possible minute before pronouncing them dead and forgotten.