A south Georgia woman told CBS' 48 Hours that she's the one who led investigators to Tara Grinstead's killer.

Brooke Sheridan told part of her story on CBS This Morning Thursday.

Grinstead, an Ocilla school teacher, went missing in October 2005, and her case became the biggest missing-person case in the GBI's history.

In February, the GBI charged Ryan Duke with killing her.

A friend, Bo Dukes, is also charged with helping Ryan Duke dispose of her body.

Sheridan told 48 Hours that her boyfriend, Bo Dukes, confessed to her that he was involved in the case.

"I felt like I was gonna be sick. I didn't know who I was staring at. I didn't know who he was," Sheridan told "48 Hours."

Dukes confided to Sheridan that his friend, Ryan Duke, had told him that he had killed Grinstead back in 2005. Duke convinced Dukes to help him dispose of her body. Both men were former students in Grinstead's history class.

Sheridan says she asked why Duke killed Grinstead, a former beauty queen.

"He said, 'That's something that only God and Ryan know,'" Sheridan said.

She says that's when she turned in the man that she loved.

"And I knew that he would probably serve the rest of his life in jail," Sheridan said. "That family's peace to me was more important than his freedom."

She says she told Dukes, "You need to confess. You need to own to what you've done and confess.' I said, 'Because that family deserves to know.'"

"He says, 'I just want her family to know,'" Sheridan said.

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Tara Grinstead (13WMAZ file photo)