(CNN) An ancient Maya document, long dismissed as a fake, is not only the real thing, but it's the oldest-known book in the Americas.

Researchers believe the Grolier codex dates back to the early 13th century. And the way they reached that conclusion is fascinating.

What is the Grolier codex?

The Grolier codex is actually a fragment of a larger manuscript, said Stephen Houston, co-director of the Program in Early Cultures at Brown University. Houston helped verify the codex's authenticity. The codex has 10 painted pages of iconography associated with Maya rituals and a calendar that charts the movement of Venus. It was probably used by priests in conducting rituals and determining when to wage war.

Wait, who were the ancient Maya again?

The Maya built an civilization that stretched through Central America, reaching its peak around the 6th century. They built great stone cities and made huge strides in agriculture, hieroglyph writing, mathematics and astronomy. But by 900 A.D. most of the civilization's cities -- located in present-day Guatemala, El Salvador, Honduras and southern Mexico -- had been abandoned, although some cities held on for a few hundred more years. Reasons for this decline remain something of a mystery for historians.

Where was the Grolier Codex found?

It was found in a cave in Chiapas, Mexico, in the 1906s by looters and sold to a wealthy Mexican collector -- before it turned up in an exhibition in New York in 1971. One anthropologist at the time declared to The New York Times that the codex would change how the Maya religion was viewed. Many other scholars weren't so convinced and deemed it a first-class forgery. It ended up back in Mexico, in the basement of the National Museum in Mexico City.

Why did some think it was a fake?

Researchers at the time theorized that forgers "had enough knowledge of Maya writing and materials to create a fake codex at the time the Grolier came to light," according to the Brown University news release . The manuscript also differed from other known Maya codices because it had more illustrations than text, which was unusual. And the fact that looters, not archeologists, found the manuscript made many scientists even more skeptical about its origins.

How did they prove it was real?

Houston's team approached the mysteries of the manuscript like investigators digging into a cold case. They put a fresh set of eyes on anything they could get their hands on related to the codex, including re-examining the practices of Maya painters and reviewing how the Maya made paper.

"It became a kind of dogma that this was a fake," Houston said. "We decided to return and look at it very carefully, to check criticisms one at a time."

Carbon dating of the manuscript's paper dates it to the 13th century, the team determined, making it older than other known Maya codices. They also believed that the thin red sketch lines underneath the manuscript's illustrations and the "Maya blue" pigments used in them were also authentic.

They note that all six items in the cave where the codex was found -- including a small wooden mask and a sacrificial knife with a handle shaped like a clenched fist -- had already been deemed authentic.

They also thought about what a forger in the 20th century would have had to do and know in order to make an acceptable fake:

"He or she would have to intuit the existence of and then perfectly render deities that had not been discovered in 1964, when any modern forgery would have to have been completed," the release says. They would have to "correctly guess how to create Maya blue, which was not synthesized in a laboratory until Mexican conservation scientists did so in the 1980s; and have a wealth and range of resources at their fingertips that would, in some cases, require knowledge unavailable until recently."

Houston said "there can't be the slightest doubt that the Grolier is genuine."