Sgt. 1st Class John H. Robertson was aboard a Vietnamese Air Force H-34 helicopter on a special operations mission over Laos when it was shot down by enemy ground fire and exploded on May 20, 1968. U.S. servicemen who witnessed the crash say there were no survivors, and his body was never recovered.
In 1976, a military review board changed Robertson's status from "missing in action" to officially deceased; his wife, who later remarried, and two young daughters moved on.
A new documentary, "Unclaimed," suggests that Robertson is still alive at 77, living in the jungle. But the U.S. government has said emphatically that the man -- a French-born, Vietnam citizen Dang Tan Ngoc -- is a well-investigated scammer.
Canadian director Michael Jorgensen's controversial documentary suggests the former Green Beret has for the past 44 years lived in poverty and is now married to a Vietnamese woman with four children. He speaks no English, according to the film, but in a reunion with his sister in Canada depicted in the documentary, she says he is for real.
The film is in English, but the director uses the services of a Vietnamese translator who immigrated to Canada when Saigon fell to the Communist government in the 1970s.
Jorgensen told ABCNews.com that what drew him to the story was his "highest regard" for those who serve in the military, who are "willing to sacrifice themselves for an ideal or just your brother. ...guys who go shoulder to shoulder with you on the battlefield."
The filmmaker asks why the American government left Robertson -- and perhaps others -- behind.
The military has investigated the case three times over the last decade, according to the Defense POW/Missing Personnel Office. "All claims and alleged live sighting reports related to Robertson have been investigated, and found to be false," they said in a statement.
Government reports obtained by ABCNews.com show, among other details, that the man's fingerprints did not match those of the fallen soldier.
The film has enraged some veterans' groups who say that the man purporting to be Robertson is a fraud and giving the film a showcase at the GI Film Festival is an insult to those who served honorably in the Vietnam War.
"... you chose to tarnish the legacy of valor of American heroes to line your pockets with thirty pieces of silver or further your agenda," wrote Don Bendell, an author and former Green Beret who served in the special forces in Vietnam, wrote to the president of the GI Festival. "You apparently do not have the balls or principle to see how you are hurting the families of veterans on this by providing a forum for a complete scam.
"You have helped con artists to give false hope to a poor unfortunate sister and survivors of an honorable hero who are clutching at any possible straw of hope that their hero might still be alive."
Bendell, 66, who has been active in the P.O.W. Network and its Fake Warriors Project, which uncover cases of "stolen valor," said that in 2006 the Robertson imposter "admitted to the government that he was a con artist and part of a fake con game.
"We know this guy is a fake," he told ABCNews.com. "I don't know if it's fame or fortune or what it is. Some people want their 15 minutes."
Bendell said the notion that Robertson could not speak English is "stupid," especially because of his training in special operations. "To make it you have to have an above average IQ and high language aptitude," he said. "The guy was no dummy...How can a person forget their native language?"
According to the government report, the crash site had not yet been excavated to look for bodies or DNA. But Bendell said, "Green Berets don't leave guys behind. There was no way others would have left him on the ground if he had been wounded."
Brandon L. Millett, co-founder and president of the GI Film Festival, has told ABCNews.com that he is aware of the controversy surrounding the film. He said the Washington-D.C.-based festival, which in the past has honored actors like Gary Sinese, will screen "Unclaimed," despite its critics.
"The festival ... will issue a statement prior to the screening to let the audience know about the objections to the film and its content -- because one of the film festival's jobs is to screen interesting stories that stimulate public debate and foster conversation," he said in a statement.
John Hartley Robertson was born in Birmingham, Ala., in 1936, according to the film. He dropped out of school to join the Army and was later chosen to be part of the elite Special Operations Group in Vietnam, MACV-SOG. He was on a war mission when the helicopter crashed in Laos.
"Unclaimed" follows the journey of Tom Faunce, a Michigan man who served two years in Vietnam and made a pledge to uphold his military oath, "leave no man behind." He returns to Vietnam four decades later and discovers a mysterious man who claims to be Robertson.
Faunce works to prove the lost soldier's identity, reuniting him with his family, sister Jean, brother-in-law Henry Holley and their two daughters, Gail and Judy. The filmmakers say the documentary provides a public forum to seek answers from the government in the handling of the case and to have conclusive DNA testing.
Jorgensen, who shot the film last year in Vietnam, Canada and the U.S, told ABCNews.com that the government has never made contact with Jean Robertson-Holley or her family. Just after the film was made, the elder Holleys were critically injured in a car accident and are now in a long-term care facility.
He wonders why the government never called the family when they were interrogating Dang Tan Ngoc, allegedly as far back as 1991 or even 1982. The last such inquiry was in 2009 when the fingerprints were analyzed, according to government reports.
"That's standard operating procedure," he said. "It's simple enough. If someone is perpetrating a hoax or is a fraudster, do your due diligence and warn people, contact the family."
Jorgensen said he spoke to a "representative" of Robertson's American wife, who had since remarried. "They just don't want to be involved," he said. "I was very respectful of that."
"I am absolutely convinced, 100 percent, that the family is convinced it is him," he said of Robertson's sister and brother-in-law. "I never set out to prove his identity. The film was about one Vietnam veteran's journey to help a guy find this man's family. ... Tom's story amazing story of what it means to be human."
As for the director of the GI Film Festival, he says "Unclaimed" is an important film that needs to be screened.
"Whether fact or fiction, "Unclaimed" is a fascinating story about a Vietnam veteran ... who dedicates himself to bringing home someone he believes to be an American GI left behind," said the film festival's Millett.
"Even if he is chasing a myth, we feel his story is compelling and worth telling, as long as it is accompanied by a disclosure to viewers about the controversy surrounding it."