At 5am on 31 August 2004, staff at the Burger King in Richmond Hill, Georgia, prepared for the day ahead. Ovens were turned on, the floor was mopped and a female employee carried a bag of refuse from the restaurant, through the empty car park and towards the small outbuilding that housed the restaurant's bins. She opened the gate and screamed. Behind the bins, naked other than his underwear, lay the body of a man.

Who that man was is a question that still hasn't been resolved. He wasn't dead, he was unconscious and would eventually come round in the nearby Memorial hospital. When he did, he couldn't remember how he'd ended up lying beside the bins, but that was the least of his problems. It's easier to record what he did remember. He believed his date of birth was 29 August 1948, thought he might have been called Benjaman and had a few blurred, fragmented memories of Denver and Indianapolis. That was it. Of the 56 years he said he'd lived, he had enough memories to fill a day.

Unidentified and uninsured, he was an administrative nightmare for the hospitals and shelters he was sent to. They kept asking him the same question: "What's your name?" Finally he made one up. Benjaman Kyle. BK. Burger King.

It's June 2010 and Benjaman Kyle sits opposite me in a Richmond Hill diner. He's articulate, witty and the only American citizen officially listed as missing despite his whereabouts being known. While the diner's customers come and go and cars zip by in the morning sunshine, the identity of the man I'm talking to is simultaneously being sought by the local police, the FBI, an American senator, DNA experts and a private investigator.

"And me," he says flatly. "Don't forget me."

Of the initial months after he was found, Kyle has few memories. One is of a conversation between doctors while he lay slipping in and out of consciousness. "They were joking," he smiles. "Calling me the Burger King John Doe. That's when I decided my name."

When Kyle awoke, he had lost his sight. "My cataracts were gone and I didn't have insurance. I couldn't see more than a couple of feet," he says. After being bounced between hospitals in Savannah, 10 miles from Richmond Hill, Kyle ended up at a men's shelter called Grace House, where he roomed with alcohol and drug abusers, and was trapped by his blindness.

"You had to leave Grace House during the day," he says. "All I could do was sit in the courtyard and wait for them to reopen. One morning I crossed the road. I couldn't see if there were cars coming." He pauses and I ask if this was a suicide attempt. "Well, I couldn't see the cars," he answers.

Eventually, nine months after he was found, a charity paid for Kyle's cataracts to be treated. With vision came enough confidence for him to approach hospital and shelter staff with a new challenge, that of discovering his identity. "I didn't recognise the man in the mirror. And I kept asking, 'Is anyone trying to find out who I am?'"

Now at least physically sound, Kyle was sent to the JC Lewis health centre. Katherine Slater was a psychiatric nurse working there. She remembers a strange new patient who claimed not to know who he was. "At first I just observed him," she says. "I guess everyone was suspicious. But he was so sweet and clever that I couldn't find it in me to doubt him. After that I got angry because no one was helping him find out who he is."

Kyle was fit to leave JC Lewis, but his lack of a social security number made that impossible. Instead he began to work there, staying for free and paid a small wage for long days assisting the patients. "He worked his days off," says Slater. "He had nothing else to do." Finally, in June 2007, she invited Kyle to move into her spare room.

Together, in the absence of official help, they started the search for Kyle's past. Slater alerted Georgia senator Jack Kingston who notified the FBI and began the tortuous process of gaining Kyle a social security number (he still doesn't have one, so can't claim benefits). She also approached Richmond Hill police department (RHPD). Since Kyle was found stripped and unconscious, Slater was "surprised" to find they hadn't conducted a criminal investigation. One was instigated when Slater made contact, three years after the event.

A local newspaper covered Kyle's story ("A Real Life Nobody") and this sparked media coverage, which peaked with an appearance on Dr Phil. Dr Phil McGraw's eponymous current affairs show is a forceful presence in American television, pulling in 7 million viewers and endlessly repeated domestically and abroad. Kyle appeared and told his story in October 2008. As a result, his memories of Denver (based mainly around the public transport system) were dated to the 80s, those of buying grilled cheese sandwiches at the Indianapolis State Fair for a quarter (25 cents) to the 50s. The show received a barrage of tips and theories. Not a single one was of any worth.

"What can you say?" Kyle says. "It was odd. I thought someone would recognise me." He believes he may have had brothers but has no other familial instincts: "No family, kids, nothing like that."

In the nearly two years since Dr Phil, Kyle and Slater have tried various angles. He tells me his DNA has been supplied to a leading genealogy specialist and his fingerprints are working their way through the FBI's system. He spends hours on the internet looking at public libraries in Denver and Indianapolis – because of his belief that he's been a lifelong reader – and he's been seeing a new psychologist. The rest of Kyle's life is spent doing odd jobs for Slater and others. "I earn between $50 and $100 a month," he says. "I hate taking people's charity, absolutely hate it, but that's the reality of my situation." Slater, meanwhile, surfs missing person websites and liaises with various official bodies searching for Kyle's identity. Of the two of them, Kyle says, Slater is the optimist. "I think all the obvious things have been tried," he says. "More and more, I think that one day it's going to pop into my head. Pop, it'll all come back."

Kyle says he has no wish to become "a famous nutcase who can't remember who he is". His media outings are carefully designed to spread coverage of his story in the hope of solving it, and when he meets people, he tends not to tell them about his situation: "You get two reactions. They want to tell you their theories or they think you're mad. Neither is much fun for me." He smiles ruefully.

Kyle struggles with dates, and in particular with the order of events, but he offers a steady, reasoned examination of the last six years. The problem is he claims to suffer from an extremely rare condition that attracts endless public curiosity. When cases have arisen, and the media have inevitably descended, the result has often been the same: a hoax.

The most notorious example of recent years was the 2007 case of County Durham man John Darwin, or "Canoe Man", who claimed to have forgotten the five years since he went missing, only for the surreal intricacies of his and his wife's life insurance scam to become international news.

Other cases are less clear but show the multiple conclusions cases of retrograde amnesia tend to offer. There was the "Piano Man" found wandering by Kent police in 2005, who drew a grand piano but four months later was identified as German care worker Andreas Grassl. In the same year, the documentary Unknown White Male publicised the story of New York-based Brit Doug Bruce who walked into a Coney Island police station in 2003 and said he didn't know who he was. Michel Gondry, director of the memory loss-themed Eternal Sunshine Of The Spotless Mind, met Bruce and publicly doubted his story.

Michael Kopelman, professor of neuropsychiatry at King's College London and expert on retrograde amnesia, says there are two views on the type of memory loss Kyle claims to have – technically called focal retrograde amnesia, meaning new memories can still be formed. "Many neuropsychologists believe it can be caused by neurological issues such as head injury, brain infection, dementia, lack of oxygen to the brain and certain types of strokes," Kopelman says. "Or it can be caused by psychological reasons, or a combination of neurological followed by psychological."

The most common psychological form of retrograde amnesia Kopelman encounters is known as the fugue state. "People go wandering off," he says. "Sometimes there's been a neurological issue in the past, but it can simply be a sudden precipitating stress such as marital breakdown, financial problems or bereavement, often combined with depression or suicidal thoughts."

When sufferers of the fugue state (recently blamed for Agatha Christie's 11-day disappearance in 1926) are caught early, Kopelman usually sees recovery in days or weeks. However, there have been reported examples of sufferers undiagnosed for several years, who are much harder to treat.

In 2002 the UK press revealed a British woman in her 50s had suffered complete memory loss in Greece and was being repatriated. Unaware of her own identity, the woman had taken the name Jezebel Blythe and her case has broad similarities to Kyle's. She had come to in an Athens backstreet in 2000, bleeding from the head and with no idea who she was or what had happened to her. After a year living rough in Greece, she was picked up by a charity and eventually returned to the UK to appeal publicly for help in ascertaining her identity (her current whereabouts are unknown).

Kopelman and his international peers debate the nature of these amnesias, often with mixed conclusions, and he acknowledges the significant element of falsehood among reported cases. "It occurs often," he says. Kopelman sees those who fake retrograde amnesia as acting out "a form of ingrained psychological role-play".

Among the medical causes for retrograde amnesia, however, there are both neurological and psychological possibilities that would apply in Kyle's case. If he was found unconscious, perhaps as a result of a mugging, then a head injury is certainly possible; meanwhile, none of the psychological triggers that might cause an extended fugue state can be ruled out, due to the lack of information about his life before 2004.

Ultimately, Kopelman concludes, "Each case must be judged on its merits." Kyle has no problem with the inherent doubt when dealing with stories such as his. "Of course," he says readily. "It sounds crazy, I know that. All I can say is I'm telling the truth."

Not everyone is convinced. Websleuths.com is a website owned and run by Utah-based Tricia Griffith, who often appears in American media to talk about high-profile cases. It may be a hobbyist message board, but the site attracts 25,000 interactive users a day, who put hundreds of hours into tracing the cases of people they are unlikely ever to have met. The opinions on websleuths.com can shape the online perception of the cases it investigates and the members have beaten the police to a solution in several instances.

In 2007 Kyle became one of the site's most popular cases until, on 3 April, Griffith posted a 350-word entry claiming that she and others had evidence Kyle wasn't beaten and so was a fraud. The 18-page discussion that followed contained two pieces of apparent evidence. The first was that Richmond Hill police had told a Websleuths member Kyle was not unconscious when found and was able to talk but chose not to do so. Second, several members said Kyle and Slater had been reluctant to provide his medical records because, they claimed, Memorial hospital had asked for a prohibitive sum to release them. "They wanted $800," Kyle says. "I couldn't afford it."

However, Harold Copus, an investigator with more than a decade's experience as an FBI case manager, who was brought in by the Dr Phil show to look into Kyle's story, supports his version of events. "I spoke to the EMTs [paramedics], Burger King staff and responding police officers. Not only was Kyle unconscious when he was found, they thought he was dead." Copus confirms that Kyle's fingerprints have been checked against the FBI's files on both criminal and civil cases, the most comprehensive search possible. Kyle also cooperated with a linguist (who agreed that a midwest background was likely) and Copus talks about taking Kyle to an "expensive doctor in Atlanta" who reviewed "Kyle's medical situation from the moment he was found".

Copus won't discuss what was discovered in Atlanta, other than to say that the final medical report "gave me a lot of confidence that he was telling the truth". Although he says he's "shocked" that no useful leads came in from Kyle's appearance on Dr Phil, Copus believes his story by "around 90%".

I contact Griffith. She's polite and well-meaning, and points out she had little choice but to draw the conclusion she did, based on Kyle's refusal to release his medical records, his sudden silence, and most of all by the advice she claims was given to a Georgia-based Websleuths member in a private meeting at Richmond Hill police station. She also forwards an email from a Memorial hospital representative who states that Kyle was given his full records at no charge in August 2008.

The RHPD and Kyle have not had a great relationship. "If I'd been in a three-piece suit and Italian shoes, they'd have looked after me a bit better," he says; he has made similar comments to local newspapers. Kyle and Slater believe a consensus was formed in the Burger King car park that he was homeless and that affected the police's attitude to looking into his case while it was warm.

I email the RHPD chief of police and get no response. I then call the switchboard and get through to the very officer the Websleuths member alleged had given the suggestion that Kyle was conscious and uncooperative when found. I tell him I have quotes from Copus stating unequivocally that Kyle was not conscious when found. "Not conscious, yes, sir," he replies with what I feel is hesitation. "Unable to speak," I clarify. "Unable to speak, yes," he replies.

The RHPD also tell me Kyle has voluntarily given them his fingerprints. Colleen Fitzpatrick, a well-regarded DNA genealogist who has worked on Kyle's case since February 2009, says he has submitted DNA samples to her and others. Her research points to two possible surnames – Davidson or Powell.

I ask Kyle about his medical records and a couple of days later he emails me. He's requested a full copy from Atlanta-based psychologist Dr Jason A King and forwards it on without editing.

In August 2008 Kyle saw Dr King, a specialist in psychological and neuropsychological evaluation. King examined Kyle extensively and reviewed all the medical records from Memorial hospital since the day of his discovery; staff there have confirmed they supplied the records that same month. King's report makes fascinating, if uncomfortable, reading.

After he was discovered in 2004, Kyle had separate periods of catatonic psychosis in September 2004 and again in October 2004. He was "diagnosed with schizophrenia" and treated with antipsychotic medication from October 2004 to January 2005. The report reveals that when Kyle underwent an appendectomy operation, it was against his will, because he was deemed "mentally incompetent to make medical decisions at this time".

Having analysed Kyle's medical notes, King subjected him to 21 separate neuropsychology tests. His conclusion was definitive. Kyle has "disassociative amnesia" which is a "manifestation of a psychiatric illness". This fits within Kopelman's definitions for psychological-driven retrograde amnesia. King states that Kyle's behaviour "is not suggestive of malingering" and the final sentence of his report ends: "To him, his lack of memory prior to 2004 is real."

I ask Kyle whether his reluctance to release the medical reports is related to a fear of the stigma of schizophrenia. "Maybe a little," he says, "but also because I think my story and the records could be worth something one day. You've got to remember I have nothing, no pension, nothing."

Does he mind his schizophrenic episodes being revealed in this article? He agrees, with a caveat: "I don't know if I agree with that diagnosis, but there you go." When I mention the Memorial hospital records, Kyle insists they had asked for payment for the records but then provided them to the Dr Phil show for free. "And not all of them," he says, "just what was felt to be necessary."

Back in Richmond Hill, Kyle and I drive to the cross section of two highways and pull into a small grid of fast-food outlets, a petrol station and a Quality Inn. The Burger King had the worst plot, farthest from the road and next to wasteland, which is perhaps the reason it closed down three years ago. The car park is potholed, the windows smashed and boarded, and a scrawled sign reads "Closed!"

"I came back a couple of times," Kyle says. "Tried to speak to the staff. They looked like they were scared of me." He stares into the empty restaurant. "They thought I was a bum. Everyone thought I was a bum."

He talks about a recent session with a psychologist, during which it was suggested the memories he had of Indianapolis, which Kyle believes is his home town, may simply have come from a visit to the city. "I'm not happy with that. I won't accept it. I need to have a home town." He's more passionate on this point than on the question of whether he's lost an entire family: parents, a wife, children perhaps. "If I had a home town, then that's a start," he says. "As for people, I have to rely on them coming to me. And no one's come."

Kyle leads me to the restaurant's rear and over to a roofless structure. He pushes open the gates and we walk into an area you wouldn't enter by accident. It's a shambles of broken glass, plastic crates and debris. Kyle walks to the corner. "I think I was here," he says, kicking at the gravel on the spot where he was discovered nearly six years ago. "They said the woman that found me was shaken up good."

It's hot in here, with the sun right above us. A silence grows. Kyle stares at the ground. I ask how being here makes him feel. "Nothing," he says simply, "absolutely nothing." But this, I gesture at the concrete and smashed glass, is where Benjaman Kyle was born. "Was he?" asks Kyle. "I can't remember."