By Matthew Davis
BBC News, Washington
Three decades since the end of the Vietnam war, the families of more than 1,800 US soldiers who went missing in action are still waiting for an end to their uncertainty.
Some families have had to put grieving on hold for decades
There is no credible evidence of American soldiers being held against their will in South East Asia, the military says.
But those who have never learned what happened to their loved ones still wrestle with unanswered questions: Were they captured? How did they die? Could they still be alive?
'He is finally home'
The family of Colonel Sheldon J Burnett - who died after his helicopter was shot down on the Vietnam-Laos border - count themselves lucky to have answers.
Earlier in April they saw the officer's remains buried with full military honours in Arlington Cemetery, the revered resting place of the US military's fallen.
Trish Burnett, one of the colonel's two daughters, told the BBC: "It was a long time coming and it just feels like this was what he deserved all along, recognition for everything he gave up.
"It means so much, just the fact that we were able to bring him back to American soil. He is finally home after all those years."
The family's campaign for the truth ended last September, when a military excavation crew unearthed Burnett's teeth and four deteriorating leg bones in a grave near the helicopter crash site.
"Until recently we could never accept in our hearts that he was dead," Trish said. "If anyone asked, we would say he was an MIA."
"When I got the call, I wasn't happy, I wasn't sad - I just cried. It was final.
"But it was like it happened yesterday, and I just found out he had died."
Lost without hope
A Pentagon team was set up in the early 1980s to investigate cases of MIAs and POWs from former US conflicts, largely in response to pressure from families and veteran's groups.
Since the end of the Vietnam War, it has accounted for 747 POWs and MIAs from more than 2,500 who went missing there.
Ann Mills Griffiths lost her brother, a US Navy reservist, after his aircraft disappeared from radar screens while taking part in a night bombing raid near the North Vietnamese coast.
"We heard on 26 September, 1966," said Ann. "I was at my parents' house, and I knew what it was as soon as the car arrived and the Navy officer stepped out."
"James was MIA, but we have never found out what happened. The last word from the plane was 'What?' and that is all we know."
Like relatives of about 500 other servicemen lost over deep water, or whose aircraft went "off-radar", Ann has little hope of ever seeing her brother's remains, or finding out about his final moments.
But for many years she has been helping others get answers from the government in her role as head of the National League of POW/MIA Families.
"It becomes a matter of principle that it is the right thing to do," she said.
"While I may never know, it makes me happy and grateful that some people can find their closure."
The US Prisoner of War/Missing Personnel Office usually sends about four field teams to Vietnam each year, each staffed by nearly 100 US experts working alongside Vietnamese officials.
The team accounting for US MIAs has an annual budget of $100m
Investigators interview former regime officials, former commanders and their troops - and seek access to sensitive documents - to identify search sites.
Vietnamese officials are reluctant to discuss how many of their compatriots are still missing as a result of the conflict. In the past the number has been estimated at 300,000.
POW/MIA Office spokesman Larry Greer said: "Our aim is to deliver the fullest possible accounting, however long that takes.
"We cannot rule out the possibility that Americans may still be in Vietnam, held against their will, but it has been 10 years since we last had a first hand account from someone who has seen an American."
He added: "The greatest satisfaction in this job is to be able to return a loved one to a family, and to give them that sense of closure they are looking for.
"Most of the people who work here are from the services - we know that there, but for the grace of God, go we."