PHILADELPHIA — Eugene Zorn wasn’t prone to exaggeration. He was a nationally recognized economist, a sober, no-nonsense man who dealt with facts and figures.

So his son, Robert, was caught off guard when, in 1980, the elder Zorn offered an unusual preface before launching into a story: “After you hear this, you may think your old man’s off his rocker.”

“I was driving and my hands tightened on the steering wheel,” Robert Zorn recalled. “He never referred to himself as my ‘old man.’”

His father then began a riveting tale that kept the 22-year-old Wharton School student up all night.

He was convinced he had solved the “crime of the century,” the 1932 kidnapping of the son of the aviator Charles Lindbergh from the family’s home in Hunterdon County, N.J., Eugene Zorn said.

‘Cemetery John’

He knew the mastermind, a mysterious figure known in the press simply as “Cemetery John,” and his younger brother, Eugene Zorn claimed. They were his neighbors in the South Bronx.

Even more incendiary, he believed he had witnessed the pair plotting the kidnapping with Bruno Hauptmann, who was convicted and executed in the murder of the Lindbergh baby.

“It was mind-boggling,” said Robert Zorn, a former software company owner whose new book, “Cemetery John: The Undiscovered Mastermind of the Lindbergh Kidnapping,” is the subject of a Nova documentary expected to be shown early next year.

“That night, I lay in my dorm room, staring at the ceiling,” he said.

Eugene Zorn didn’t think about the men until 1963, when he came across a True magazine in a Dallas barbershop.

The cover story detailed the case against Hauptmann and mentioned suspected accomplices, including Cemetery John, who had never been arrested. Zorn began to piece together three-decade-old memories.

He was 15 in 1931 when neighbor John Knoll, a German immigrant, offered to take him to Palisades Amusement Park across the Hudson River from Manhattan.

“Waiting for John (at the park) was his youngest brother, Walter, and a third German-speaking man they called Bruno,” Robert Zorn said.

Eugene Zorn “heard them talking about Englewood,” N.J., his son recalled.

After reflecting on the long-ago conversation, he became convinced he had witnessed the kidnappers’ early planning, Robert Zorn said.

John Knoll was “Cemetery John,” Eugene Zorn told his son. He thought the unfamiliar Bruno was Hauptmann.

And Englewood was where the Lindberghs were staying with family while their home, near Hopewell, was being built in 1931.

Robert Zorn researched the case with his father and promised him on his deathbed, at age 90 in 2006, to “someday tell the story to the world.”

Originally, said the younger Zorn, he knew only what most people knew about the highly publicized crime, what the newspapers and history books reported.

“Little Lindy,” Charles Lindbergh Jr., was placed in his crib at 8 p.m. on March 1, 1932, and was discovered missing at 10 p.m.

His father, known as “Lucky Lindy,” discovered a handwritten ransom note on a radiator in the nursery. It was filled with spelling and grammatical errors, but was clear enough: Lindbergh was to pay $50,000.

Authorities put up a reward of $25,000 for the child’s return, and the Lindberghs offered $50,000 more.

A meeting

A retired teacher and Bronx personality, John Condon, added $1,000 to the reward and reached out — through a letter in a local paper — to the kidnappers.

Unknown to police, a meeting between Condon and a man identified as a kidnapper took place at a Bronx cemetery. The man, who provided only his first name, came to be known as Cemetery John.

A police sketch of the suspect, based on Condon’s description, closely resembles a photo of John Knoll, obtained by Robert Zorn in 2010. A fleshy lump on Cemetery John’s right thumb matched one seen in another photo of Knoll. And writing on a ransom envelope addressed to Condon is strikingly similar to samples of Knoll’s script in Zorn’s possession.

Condon met Knoll again at another Bronx cemetery and gave him $50,000 in gold certificates and regular currency, provided by Lindbergh.

The suspect passed him a note with the location of the 20-month-old child: Little Lindy was being held on a boat on Martha’s Vineyard. The vessel was never found, and the infant’s body was discovered May 12, 1932.