SPARTANBURG, S.C. -- A South Carolina man convicted of killing seven people says he has more victims who have not been discovered.

The Herald-Journal of Spartanburg reports that, in an eight-page letter, Todd Kohlhepp wrote that he tried to tell investigators and informed the FBI, but he said "it was blown off." He also wrote, "At this point, I really don't see reason to give numbers or locations."

Don Wood, chief division counsel with the FBI's Columbia office, said the agency has a pending investigation, but wouldn't comment specifically on what the FBI is doing.

Kala Brown WSPA

The 46-year-old Kohlhepp pleaded guilty in May to seven counts of murder for killings that took place over more than a decade, all as he ran a real estate business. He was sentenced to life in prison.

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Kohlhepp was arrested when investigators searching for a missing couple found a woman "chained like a dog" inside a shipping container. The woman, Kala Brown, had been raped and locked inside the container for more than two months after Kohlhepp shot and killed her boyfriend, authorities said. Charles David Carter, 32, was the last of the seven murder victims.

CBS News does not normally identify victims of sexual assault, but Brown has spoken publicly about her ordeal. She said on the "Dr. Phil" show in February that Kohlhepp raped her daily during her captivity and bragged that he was good at killing, claiming that his victims numbered nearly 100.

Brown said she did what she had to do to survive.

"He told me as long as I served my purpose, I was safe," Brown told host Phillip McGraw.

Kohlhepp's mother Regina Tague, in an emotional interview with "48 Hours," insisted her son is "not a monster."

"I want [Brown] to know how sorry I am," Tague told "48 Hours." "And I think Todd is too. Because he didn't wanna hurt her, he just didn't know what to do."

Tague was found dead in her home in April. The coroner said she died of natural causes, CBS affiliate WSPA reports.