The Lindbergh mystery: Could America’s most famous crime be solved at last?

The Lindbergh kidnapping trial of 1935, in which one man was sentenced to death for the killing of an infant, is still considered to be America’s most puzzling mystery. Can a new book resolve the infamous inconsistencies of the crime? Lifelong obsessive Rupert Cornwell investigates.

It was, beyond doubt, the Trial of the 20th Century (and, let it be said, nothing yet in the 21st century has come remotely close). It might also be described as the Circus of the Century, a media extravaganza in lousy taste, followed avidly in every corner of America and in most of the world beyond.

Above all, however, it remains a mystery for the ages. Was justice really done back in February 1935, when Bruno Richard Hauptmann, an immigrant German carpenter, was tried, convicted and sentenced to death in Flemington, in rural New Jersey, for the murder of Charles Lindbergh's 20-month-old son, Charlie Lindbergh Junior, almost three years before?

For eight decades the question has nagged and fascinated. For one thing, Hauptmann, even when offered the chance of saving his life by telling the truth, protested his innocence until the very moment he went to New Jersey's electric chair. And if he was involved, could one man have carried out so audacious and elaborate a crime, acting alone? To kill a president during a public open-top motorcade in Dallas, with a rifle, is a matter of a few seconds – simple compared to organising and carrying out the kidnapping of a child from inside that child's own house, conducting ransom negotiations and then returning the infant (or, in this case, disposing of his body).

But now the Mystery of the Century may just have found an answer. It has been provided not by an overlooked piece of ancient evidence, or by the wizardry of modern forensics, but by a father's memories of one strange day in 1931, recounted to his son almost half a century later. Eugene Zorn, then a young boy growing up in the Bronx, New York, believes he witnessed the conspirators plotting their crime. Gene Zorn died in 2006, but Robert Zorn took up the trail. The result is his book Cemetery John, perhaps the most intriguing and plausible version thus far of the Lindbergh kidnapping.

America has seen its share of famous trials over the past 100 years: from Sacco & Vanzetti to OJ Simpson, from the Rosenbergs to Charles Manson. But nothing matched the Lindbergh case and the trial of Bruno Richard Hauptmann. HL Mencken called it "the greatest story since the Resurrection. The crime horrified the entire world, drawing parliamentary statements from the prime ministers of Britain, France, Japan and China. Lindbergh's solo flight across the Atlantic in 1927 had made him the most famous man in the world. On the final weekend of the trial, 60,000 sensation-seekers, from New York celebrities to prostitutes looking for easy business, crammed into a town with a normal population of 5,000. Reporting on the case were the likes of Damon Runyon, Ford Madox Ford and Walter Winchell, inventor of modern US tabloid journalism.

Little Charlie had been abducted on a blustery, rainy night in March 1932 from the secluded house in Hopewell, New Jersey that the Lindberghs, tired of a celebrity existence in their mansion at Englewood just across from Manhattan, were preparing to move into permanently. The window in the empty nursery was open. On the sill was a ransom note, written in broken, German-inflected English, demanding $50,000 (equivalent to £468,000 today). Other notes followed, all postmarked from New York.

John F Condon, a retired Bronx schoolteacher known as 'Jafsie', emerged as the Lindbergh's intermediary with the kidnappers (it was universally assumed a gang was responsible). After a meeting in a local cemetery between Jafsie and a man with a German accent who gave his name as 'John', the ransom was handed over. But on May 12, 1932, a lorry driver who had stopped to relieve himself stumbled across the infant's decomposing body in a roadside wood near the Hopewell residence. A kidnapping case had become a murder.

For more than two years the trail went cold. Then, in August 1934, one of the marked gold-certificate bills used to pay the ransom was traced to Hauptmann, a German carpenter who lived in the Bronx with his wife Anna and 11-month-old son Manfred. Hauptmann was arrested, interrogated and charged, and tried in Flemington, seat of Hunterdon County where the crime was committed. His appeals exhausted, he was executed on April 3, 1936.

But two great uncertainties hung over the case, even then. Why would Hauptmann maintain his innocence, even when offered mercy if he told all? Then there was the matter of accomplices. Until his arrest, the Lindberghs, the police and the media all believed a gang had carried out the crime. Two sets of footprints were discovered at the Hopewell house, as well as a discarded ladder. Even the ransom notes talked of "we".

But once a solid suspect was in hand, the investigation was effectively closed down. Above all, police wanted to wrap up a case that had dragged on for two and a half years, causing them huge embarrassment, and which had already cost $1.2m (£11m today). In court, Condon dropped his previous misgivings to identify Hauptmann as 'Cemetery John', while Lindbergh claimed to identify a voice he had heard utter words while waiting in a nearby car as the ransom was paid, as that of Hauptmann. Naturally this remarkable feat of memory went unchallenged. In the America of the time, the word of the hero 'Lucky Lindy', now smitten by terrible personal tragedy, was unassailable.

Undeniably, the Hauptmann case was a less than splendid advertisement for US f justice. Whether or not evidence was planted, he was very harshly interrogated, without the presence of a lawyer. His defence lawyer in Flemington was an over-the-hill alcoholic in the pay of Hearst Newspapers, while anti-German prejudice (after the First World War) was rife. In fact, the circumstantial evidence is overwhelming – from the $15,000 of ransom money discovered in his garage to wood from his attic used in the kidnap ladder – that Hauptmann was at least involved in the crime. On the witness stand, the defendant was caught in lie after lie by the state prosecutor David Wilentz.

Ever since, argument has raged over Hauptmann's guilt. The case has inspired two films and more than a dozen books: some depicting Hauptmann as the sole kidnapper, some suggesting that the tiny corpse was not Lindbergh's son (over the years at least 15 imposters have surfaced) and one claiming that the killer was the aviator himself. Ludovic Kennedy's 1985 bestseller, The Airman and the Carpenter, argues that Hauptmann was framed by perjured evidence, the innocent scapegoat in a case the authorities were desperate to see the back of. Eugene Zorn, however, kept quiet for decades; until March 1980, when he told his son Robert an extraordinary story.

By then, Zorn had made a career as an eminent economist. He was an unassuming man, trained to respect facts and not normally given to flights of fancy. But, that 1980 day, as the pair were driving to the airport, he turned to his son: "After you hear this, Bob, you may think your old man's off his rocker."

It had all happened when Gene Zorn was a teenager, growing up in the Bronx, in a modest rented apartment with his parents. Three doors down, in a $10-a-month rented room, lived a German grocery-store worker named John Knoll. Knoll was 10 years older but the two had become friendly, partly through a shared interest in stamp collecting.

And so, one summer day in 1931, John Knoll invited 15-year-old Gene on a trip to Palisades amusement park, just across the Hudson river from New York City. After splashing in a pool, Gene rejoined John who was talking with two men, one of whom he recognised as John's brother Walter. They were speaking in German, a language Gene didn't understand. He did pick up two words, however. One was "Englewood". The other was "Bruno" – the name of the third man, whom Gene had never seen before.

At that point, Knoll brusquely told Gene to return to the Bronx by himself. Surprised and upset, he did so, but never forgot the incident. And years later, in 1963, it all came flooding back. Sitting in his barbers one afternoon waiting for a haircut, Gene Zorn picked up a copy of True Magazine that was peddling the latest crackpot theory about the Lindbergh kidnapping. He read about Englewood where the Lindberghs lived, about 'Bruno' Hauptmann, about the mysterious individual called 'Cemetery John', both of them who spoke with German accents. He remembered the bizarre episode at Palisades Park. He remembered how his mother (Robert's grandmother) hadn't liked John Knoll, referring to him as "the crazy Dutchman". And a thought struck Gene Zorn that would never disappear: had he been present as the Lindbergh conspirators discussed their plot?

Quietly, he researched the case, visiting the Library of Congress in Washington, devouring every word he could find on the subject. He even approached Lindbergh himself, with a letter delivered by an intermediary, a former Secretary of the Treasury whom Zorn knew and who sat on a corporate board with the aviator. But Lindbergh rebuffed him, for easily understandable reasons. After so much hounding by the media, so many hoaxes and so much grief, why re-open the past once more? Finally, in March 1980, Gene Zorn told his son. Together they pursued the investigation, though as something of a sideline. And before his father died, in December 2006, Robert promised that he would make it his mission to establish the truth. The result is Cemetery John.

A successful software entrepreneur until he devoted himself full-time to the Lindbergh kidnapping, Robert Zorn is an affable, instantly likeable man, but not at first glance one of nature's born sleuths. Moreover, unlike many who have held forth on the Hauptmann case, he approached it with an open mind. "It was perfectly possible the meeting at Palisades Park had no nefarious purpose," he told me. "If I'd found anything disproving my dad's suspicions, if I learnt that John Knoll was a really great guy, I would have dropped the whole thing. That's what my father would have wanted." Instead, exactly the opposite happened.

Fact after fact tended to support Gene Zorn's theory. Not only is the physical resemblance striking between the artist's impression based on Condon's description and the real John Knoll. Condon also remembers a fleshy lump on the base of Cemetery John's left thumb, confirmed by contemporary photos of Knoll. Hauptmann did not possess such a lump – which is why Condon said after the initial identity line-up following Hauptmann's arrest, "No, he is not the man".

The man in the cemetery identified himself as "John". The first ransom note was posted from where John Knoll lived and worked. f State-of-the-art handwriting recognition technology suggests a high likelihood that Knoll wrote the ransom envelopes. The baby's sleeping suit returned by the kidnappers was wrapped in paper of the kind used for deli foodstuffs. Both John and Walter were deli clerks, preparing sandwiches and the like for customers. As for the $15,000 hidden in Hauptmann's garage, it is slightly less than a third of the total ransom. Crude arithmetic suggests a three-way split of the proceeds.

Like Hauptmann, too, Knoll started to spend money after the ransom was paid over. He shelled out the small fortune of $700 for first-class tickets on the liner Manhattan for his wife and himself. They left three weeks before the trial began, while he sailed back alone, leaving on the very day the trial ended – when it was clear, Zorn argues, that Hauptmann had not betrayed him on the witness stand (reporting on the case in Germany, needless to say, was comprehensive).

After the ransom was paid, he began to shower Gene Zorn with expensive commemorative covers, some with an aviation theme, one even featuring Lindbergh himself, all acquired on the wages of a deli clerk. There is no proof Hauptmann and Knoll even knew each other. But there is a possible connection – Zorn's landlord who knew the Knolls, and whose family had lived very close to the Hauptmanns in the small Saxony town of Kamenz.

Psychologically, too, Knoll is a good fit. Zorn has dug deep into the family history and spoken to surviving relatives. His suspect emerges as reckless, cruel and manipulative, fond of the flamboyant gesture, craving for celebrity. One ransom note spoke of the kidnapping as a "world affair". Clearly, its author or authors relished the impact of their deed. There were easier and richer targets than America's national hero in the Depression era, when kidnapping for money was a fairly common crime, but none as famous.

Would the assembled evidence have persuaded a grand jury to indict Knoll – let alone clear the hurdle of "beyond reasonable doubt" required for a conviction at trial? Maybe not. A decent defence lawyer could make the coincidences appear just that, each susceptible to an innocent explanation. And Zorn's construct does not resolve every riddle of the Lindbergh case. How, for instance, did the kidnapper or kidnappers know that because the baby had been sick, the Lindberghs would be staying on at Hopewell for the night of the kidnapping, before returning to Englewood? Theories that someone in the household (the Lindberghs had no less than 29 servants) acted as an accomplice have long abounded. Nothing, though, has ever been proved.

But the kidnapping and murder of little Charlie Lindbergh is not a court case any longer, it is history. The main actors are long gone. John Knoll died in 1980, his brother Walter in 1962. Condon, the one person who set eyes on the kidnapper, died in 1945. Charles Lindbergh died in 1974, although Ann Morrow lived on until 2001. Anna Hauptmann went to her grave in 1994, campaigning until the last to clear her husband's name. Their son Manfred, born 18 months after the kidnapping, is now approaching 80 and has never spoken publicly of the case.

The only forensic avenue still unexplored is DNA testing of the kidnap ransom envelopes for traces of saliva. The envelopes are in good condition; I asked Theodore Kessis of Appled DNA Resources whether 80 years on, they could hold a clue. "Absolutely," he replied. But, Kessis noted, comparative samples could now only be provided by surviving relatives of Hauptmann and Knoll, making it "possible but far from certain" they would provide a definitive answer.

For historians, however, standards of proof are less exacting. Their task is to reconstruct events as plausibly as possible from the information available. And you don't have to buy the more exotic arguments of the profilers – for instance, that Gene Zorn was deliberately used by Knoll as an unwitting archivist of the crime – to accept that of the revisionist theories in the Lindbergh case, the one advanced by Zorn's son is the most persuasive.

No single fact or combination of facts proves that John Knoll was Cemetery John. But the fragments Zorn has uncovered form a mosaic whose pattern is clear. "You can only go on for so long calling one coincidence after another a coincidence," he says."Life is simply not that coincidental." For the former FBI agent Ed Sulzbach, "like the physical profile, the behavioural profile of the ringleader of this crime matches the personality profile of John Knoll." Hauptmann might have been five years older than Knoll, and of better circumstances, but every indication is that he was "a common criminal and not a leader", Sulzbach argues.

And there, for now, matters rest. Flemington itself has plainly seen better days. Main Street has the fusty, neglected feel of many old American towns ringed by new highways and shiny suburban shopping malls. The old Union Hotel across the road, where Damon Runyon and Walter Winchell held court, that for a few weeks in early 1935 was the news centre of the world, stands empty and abandoned. In 1996, trials and other legal business moved down the street to the new Hunterdon County Justice Centre. But Flemington's ghosts are eternal.

"They think that when I die, the case will die," Hauptmann said shortly before his execution. "They think it will be like a book I close. But the book, it will never close." In that respect at least, he was telling the truth. Zorn's book has shaken up the enduring cottage industry spawned by the Lindbergh kidnapping. A new US public television documentary, incorporating the findings of Cemetery John, airs in early 2013, and there is talk of a new film.

On two successive late September evenings in Flemington, 150 people turned up to hear Zorn give a talk about his book in the handsome old courtroom with polished wooden benches like church pews, its oil portraits of judges past, and the same chair from which Hauptmann, Lindbergh and 'Jafsie' testified all those years ago. The night I attended, Zorn ended by telling his audience how John Knoll died in 1980. It turns out that he fell off a ladder, hit his head and never recovered. A belated kind of justice? Or just another coincidence?