SAPULPA, Oklahoma - The Oklahoma State Bureau of Investigation says agents have unearthed the remains of three people in Pawnee County.

Grover Prewitt, 50, of Bristow, has been arrested, but investigators expect more arrests in the coming days.

OSBI says it’s treating the deaths as triple homicide and Grover has been charged with accessory to first-degree murder for concealing the fact that three people were murdered and buried on family property, according to an affidavit.

Grover is being held on $500,000 bond at the Creek County Justice Center.

It's a complicated case, one that, according to an affidavit, involves a family secret that's been kept for two decades.

SkyNews 6 was flying above Pawnee County on April 16, the day investigators discovered the bodies of Wendy Camp, her 6-year-old daughter Cynthia Britto and Camp's sister-in-law Lisa Kregear.

“Twenty-one years ago, these two women and this 6-year-old child were murdered and buried under 8 feet of dirt,” Lincoln County District Attorney Richard Smothermon said, pointing to a photo investigators released of the grave near Jennings.

Investigators are still waiting on DNA results, but they say clothing found on the skeletal remains matches what the three were last seen wearing.

Camp's sister, Aisha Hashmi, says the way they were thrown into the ground was horrific.

“They never confirmed how they were killed, but you can plainly see the bullets,” Camp’s sister Aisha Hashmi said. “It just brings more questions of ‘how did they go? Who did they shoot first?’ It was really hard to see [the photo].”

The three went missing in May 1992.

Camp's ex-mother-in-law, Beverly Noe was reportedly the last to see them alive.

Foul play was always suspected, but no arrests were ever made.

Hashmi says there's no doubt in her mind who murdered her family.

“It was Beverly who probably pulled the trigger, out of all of them, Beverly is the only one I can say without a doubt in my heart who killed them,” Hashmi said.

Grover, who is Beverly’s brother, been charged with helping their mother, Ida, hide the bodies and evidence of the crimes.

Pawnee County land records show Grover owned the land where the bodies were buried, but he sold that land to his mother around the time the three went missing.

According the affidavit, the hole the bodies were found in was dug for a septic tank.

Grover told investigators his mother asked him to fill the hole, but said he didn't look in it, because he was afraid of what he would see.

The affidavit says Grover told detectives that he thought Ida and Beverly killed the three victims.

Investigators said a tip led them to the Prewitt's property.

According to the affidavit, Grover showed investigators where to dig.

Ida died in 2011.

Beverly is still serving probation for conspiracy and arson convictions, in which she and her mother both served prison time. The two women were imprisoned in 2007 for intentionally burning down their Bristow home to collect more than $83,000 from an insurance company.

OSBI agents believe there are more people involved and the agency is offering a $5,000 reward for information that leads to the arrest and conviction of those responsible for the murders.