PAWHUSKA, Oklahoma - New York Times best-selling author David Grann set out to tell a story, almost forgotten with time.

"So many Americans, and I include myself among them, knew nothing about this history," said Grann. "We had in effect excised it from our conscious."

It's a story almost too unbelievable to be true. A conspiracy, full of secrets, greed and scandal, that helped launch the FBI.

"Part of this journey was trying to kind of chase history and try to, hopefully, capture it," Grann said.

His novel, Killers of the Flower Moon, details the 'Reign of Terror' in Osage County. Grann describes how dozens, if not hundreds, of wealthy Osage Indians were murdered during the 1920s after oil deposits were discovered on their land.

"One of the things that makes these murders so painful is that many of them never had resolution," said Grann. "So many of these families were haunted by these cases."

The real life drama captivated readers. It was one of Time Magazine's top 10 non-fiction books and, now, Martin Scorcesee is adapting Killers Of The Flower Moon into a major Hollywood movie, starring Leonardo Dicaprio and Robert De Niro.

Since a lot of the details were lost with time, Grann traveled to Pawhuska to do research and meet with tribal members.

"David called me and told me he was going to do an article for the New Yorker and asked if I would help him," remembered Osage Member Kathryn Red Corn.

At that time, Red Corn was the director of the Osage Tribal Museum. She agreed to meet Grann and help with his research.

"We worked for several months, close to a year," Red Corn says. "He called me and said, I guess you know this has turned into a book. There was so much information."

Red Corn, like many descendants still living in Osage County, grew up knowing all about the murders. Her grandfather was poisoned by his wife. Her grandmother, on the other side, died during an experimental c-section performed by a doctor from out of state.

"One of the allottees told me one time, she said 'you know your grandmother was murdered'?," remembered Red Corn.

Grann said he soon realized Red Corn's story was one of many, and each person he met had a story that needed to be told.

"These crimes were in effect inheritance schemes. They involved people marrying into families, pretending to love you, while systematically plotting to kill you over years," says Grann.

Billie Ponca is related by marriage to one of the Osage murder suspects.

"Bryan Burkhart married my great aunt. He was involved with the Anna Brown killing," says Ponca.

Danette Daniels says her grandparents had one of the last arranged Osage marriages in Fairfax, which is where many of the killings happened. She remembers as a child, her family didn't really talk about the murders.

"Everybody knew. But it was just so heart wrenching we didn't really discuss it openly," said Daniels.

For years, Grann would go back to Osage County, to learn more and finish his book.

"One of the things you hope with a story like this is that it will become a part of our conscious, part of our national narrative," Grann said. "So many people got away with it. And so now the world knows."

Production on the movie starts next month. Producers will hold a casting call on November 9 & 10 for Osage members who want to audition for speaking and background extra roles.