The upcoming film Killers of the Flower Moon, directed by Martin Scorsese and starring Leonardo DiCaprio, has a disturbing true story at its core.

The movie boasts a talented cast, including Robert De Niro, Brendan Fraser, and Lily Gladstone.

However, the film's focus on real-life events and people during a dark period called the "Reign of Terror" is what gives it weight and significance.

The movie is based on a non-fiction book of the same name published in 2017. The book tells the story of the violent killings that took place during white settlers' attempt to take over an Osage family's land in Oklahoma, which was discovered to have some of the world's most valuable oil fields beneath it.

"That this conspiracy of theft, terror, and genocide helped launch J. Edgar Hoover's FBI deepens the narrative's moral complexity."

But what occurred in reality, beyond what was documented in the book or portrayed in the upcoming film?

From the 1910s to the 1930s, many Osage Native Americans were killed in Osage County, Oklahoma.

These murders targeted at least 60 healthy people who held Osage headrights, as reported by the Osage Nation Government page. Unfortunately, the investigations into these deaths were never solved.

Later, it was suspected that other deaths during the same period, referred to as the 'Reign of Terror' from 1921 to 1926, could have been covered up.

Those responsible were likely motivated by the land and wealth of certain Osage members. As per the Oklahoma Historical Society, in the 1920s, some Osage members became extremely wealthy when oil was discovered beneath their land.

In the movie, DiCaprio portrays the character of Ernest Burkhart, who was advised by his uncle, William K. Hale (Robert De Niro), a wealthy cattleman, to marry an Osage member named Mollie (Lily Gladstone).

However, in reality, Burkhart and Mollie lived with Mollie's mother, Lizzie Q.

Lizzie's other daughter was also found dead in May 1921, with a bullet wound in the back of her head. However, her case went unsolved as she had no known enemies.

Similarly, when Lizzie died from suspected poisoning, the case remained unsolved.

More of Mollie's family members turned up dead but Mollie and Burkhart survived, inheriting a large fortune.

However, their luck ran out when Burkhart and his uncle, Hale, were arrested in 1926 on suspicion of Hale having intimidated and bribed individuals to carry out the killings.

Hale was eventually convicted three years later, but only for one of the murders.

Hale and Burkhart had already begun poisoning Mollie when they were arrested, but fortunately, she recovered. The fate of Mollie's family is just one example of what happened to many Osage families during the 'Reign of Terror' period.

Numerous actors involved in the production of the movie have recognized the significant importance of the true events on which it is based.

Lily Gladstone posted on Instagram: "I consider it a true gift and great responsibility to be trusted with Mollie Burkhart, and will hold her preciously with both arms, close to my heart."

"My most profound thanks to Osage Nation, it is a remarkable gift to be welcomed by you, and to be able to tell this story."

An official release date for Killers of the Flower Moon, which is set to be distributed by Apple TV+, has not been publicly announced yet.