Josué Sáenz did not know where the plane had taken him.

It was the mid-1960s, and he was landing at a remote airstrip tucked between the rugged peaks of the Sierra Madre de Chiapas and Tortuguero, Mexico — at least, according to the account he would later provide, one that skeptics dismissed as fantasy. Sáenz, a former Mexican cabinet member, collected pre‐Columbian art, which was why he was currently following two looters into the wilderness.

The men had convinced Sáenz of an exceptional item’s existence. But they did not quite trust him. To ensure Sáenz could not retrace the flight path, he said the men hid the airplane’s compass beneath a cloth.

What the pair had uncovered late in 1965 was a book — or, more specifically, less than a dozen painted fig bark sheets ripped from an ancient astronomy manuscript. The looters said they had found the pages preserved in a dry cave, heaped with a wooden mask and a sacrificial blade.

The dubious circumstances of the book’s discovery would haunt it for decades. But new research, published recently in the journal Maya Archaeology, aim to put the mystery to rest. The manuscript, anthropologists at Brown University, Yale University and the University of California at Riverside announced Wednesday, is the oldest surviving text written in the Americas, dating back to the early 13th century.

Sáenz may not have known its significance when he saw it, but the collector decided to purchase the manuscript anyway. In 1971, he allowed it to be displayed at a bibliophilic society, the Grolier Club, in New York. To anthropologists, the book came as a shock — it appeared to be text of a Maya calendar. If authentic, it would be only the fourth such text discovered in the world.

The Grolier Codex, as it would come to be known, was different from the other three manuscripts. The Dresden, Paris and Madrid Codices were booty from the Spanish conquest, sent back to Europe at some time during the 16th century. The Grolier Codex remained hidden in the Americas for far longer. It also had fewer hieroglyphs and more weight on illustrations, as the new analysis noted.

Like the others, the Grolier Codex detailed the Maya’s interest in the celestial movements of Venus. When the codex was on display in the club in 1971, Michael D. Coe, a Yale University anthropology professor, told the New York Times the text could revolutionize the way scholars perceive the ancient Maya religion.

“The codex shows us for the first time that the Mayas considered all four phases of the Venus cycle to be equally malevolent and threatening to human welfare,” Coe said, rather than just the initial phase. He carbon-dated a stamp-sized square of blank bark paper, also found in the cave, and pegged the Grolier to the year 1230, give or take a hundred years.

The codex’s existence, Coe said, was “nothing less than a miracle.”

To other experts, the codex’s existence was nothing less than a fraud.

One of the most outspoken critics was British archaeologist Eric Thompson, who argued the Grolier codex was missing references to key Maya omens. Though he did not dispute that the fig paper itself was ancient, blank paper is more common than codices; Thompson could not rule out that a fraudulent artist had painted on an ancient canvas. The codex showed possible signs of tampering, too, with suspicious cut marks that may have been an attempt to add years to the book.

Plus, there was the issue of the looters — the Grolier Codex, after all, did not have the most reliable of origin stories.

“It is possible that it is a late colonial period codex, but more likely it was painted as recently as the 1960s when looters realized the blank paper they found in a cave would be much more valuable with painted designs,” wrote Florida Museum of Natural History’s Susan Milbrath in a 2002 paper, echoing Thompson’s concerns.

Subsequent scientific investigations were hampered by the illicit nature of how the codex ended up at the Grolier Club. It is illegal for such artifacts to leave Mexico without permission — and so the codex swiftly vanished after the bibliophiles’ show.

More than a half-decade later, in the wake of a joint Mexico-U.S. customs investigation, the Grolier Club’s president admitted to smuggling it in his suitcase, sandwiched between pieces of cardboard and rubber bands. In the wake of the 1977 inquiry, Sáenz coughed up the codex in Mexico, giving it to the National Museum of Anthropology.

Years passed. Beyond an inconclusive analysis of the Grolier Codex’s ink in 2008, there was little new research to shed light on the manuscript.

That is, until now, according to a team of anthropologists who worked with Coe.

“There can’t be the slightest doubt that the Grolier is genuine,” Brown University’s Stephen Houston, an expert on Mesoamerican culture, said in a news release on Wednesday.

The researchers say they proved the text was real by systematically ruling out fraud. Among the most damning evidence against a forgery, the scientists said, are the “perfectly render[ed]” Maya deities unknown to archaeologists in 1964. Plus, a forger would have to recreate the formula for blue Maya ink, which Mexican scientists would only discover in the 1980s. Whoever drew the Maya weaponry, the researchers wrote in their paper, did so in a way that “not a single detail fails to ring true.”

Convinced of its authenticity, the scientists make a final claim: Radiocarbon dating puts the Grolier Codex before the Dresden Codex, which was previously considered to be the most ancient of Maya texts. If so, America has a new oldest book.