The details came to light only when police rescued a woman chained at the neck in a metal storage container on Thursday, authorities said.
Todd Kohlhepp accepted responsibility for an unsolved 2003 massacre the day before the 13th anniversary of the deaths that stumped authorities, said Sheriff Chuck Wright.
Kohlhepp, 45, confessed to killing the owner, service manager, mechanic, and book-keeper of Superbike Motorsports, a motorcycle shop in Chesnee, in south Carolina.
“God answered our prayers. If it wasn’t for Him answering our prayers and Todd talking to us, I don’t know that we’d ever solve that case,” Sheriff Wright said.
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Sheriff Wright said that on Saturday Kohlhepp also showed officers where he says he buried two other victims on his 95-acre property near the town of Woodruff in the state, in addition to the body found on Friday at the site.
One victim was identified as 32-year-old Charles Carver, the boyfriend of the woman found in a the metal container.
Mr Carver and the woman went missing around August 31.
Their last known mobile phone signals led authorities to the property.
Mr Carver died of multiple gunshot wounds and an anthropologist is helping determine how long he was buried, the coroner said.
The sheriff said it was possible more bodies would be uncovered.
The wife of one of the 2003 victims said detectives told her Kohlhepp was an angry customer who had been in the shop several times.
Melissa Ponder said she was resigned that her husband Scott’s death would never be solved before getting a phone call on Saturday from one of the case’s original detectives.
The Superbike killings stunned the Chesnee community, with rumours that they were committed by a Mexican drug gang or part of a love triangle.
Ms Ponder said she was glad the rumours were untrue.
“It isn’t closure, but it is an answer,” she said. “And I am thankful for that.”
Kohlhepp was released from prison in Arizona in 2001.
Aged 15, he was convicted of raping a 14-year-old neighbour at gunpoint and threatening to kill her siblings if she called police.
Kohlhepp had to register as a sex offender.
He went on to get a South Carolina estate agent’s licence in 2006 and build a firm.
Sheriff Wright said “it’s strange” that Kohlhepp managed the pretext of a normal life for so long.
Scott Waldrop, who has lived next door to the Woodruff property for nearly 22 years, said “he didn’t seem like a threat”.
Mr Waldrop said when he saw the container where the woman was found, it was full of bottled water and canned goods.
After buying the property two years ago, Kohlhepp immediately started putting a chain-link fence around it.
Kohlhepp paid Mr Waldrop to put no-trespassing signs, cut trees for him and other odd jobs around the property.
Kohlhepp also installed deer cameras and put in bear traps throughout.
“I was the only one he let over there, I think because I laughed at his jokes and listened to him,” he said.
“I just hate to know somebody who’s done something like this.”