An estate agent who allegedly kept a woman chained like a dog after burying her boyfriend in a shallow grave has confessed to being a serial killer who has claimed the lives up to seven victims, a US sheriff has told reporters.

Todd Kohlhepp, 45, was arrested after police found 30-year-old Kala Brown “chained like a dog” inside a shipping container on his 95-acre (38-hectare) tract of rural land outside the town of Woodruff, South Carolina, it was reported.

Police are said to have rescued her after hearing a banging sound when they arrived at Kohlhepp’s property with a search warrant having tracked Ms Brown’s last known mobile phone signals to the estate agent’s land.

Officers are understood to have found Ms Brown with chains around her neck and ankles, trapped inside the padlocked container. She had been missing since 31 August, and allegedly told police she had been in the container for two months.

She is also said to have told officers that Kohlhepp, a registered sex offender, shot her boyfriend Charles Carver, 32, several times in the chest in front of her.

After discovering Ms Brown on Thursday, police reportedly found Mr Carver’s body in a shallow grave on Kohlhepp’s land on Friday.

Then, late on Saturday, Spartanburg County Sheriff Chuck Wright told a press conference that Kohlhepp had led police to two more gravesites and confessed to killing up to seven people.

Four of his victims, it was reported, came in a single quadruple killing – the Superbike Motorsports massacre that had baffled police and investigators for 13 years.

The 6 November 2003 shooting claimed the lives of the owner, his book-keeper mother and two employees of Superbike Motorsports, a motorcycle shop in Chesnee. It was Spartanburg County’s first quadruple homicide.

Despite a series of rewards being offered, the killer of store owner Scott Ponder, 30, his mother Beverly Guy, 52, service manager Brian Lucas, 29, and mechanic Chris Sherbert, 26, had never been found. Investigators had struggled to come up with even a possible motive.

On Saturday, however, Sheriff Wright told reporters: “There’s no wondering any more. He told us some stuff [about the massacre] that nobody else ought to know.”

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It also emerged that Kohlhepp has been a registered sex offender since he was convicted in Arizona in 1987 of raping a 14-year-old neighbour at gunpoint and threatening to kill her siblings if she called the police.

It was reported that court documents show that during the rape, committed in 1986 when Kohlhepp was 15, he duct-taped his victim’s mouth and tied her hands together.

Kohlhepp served 14 years in jail before being released in 2001.

He had to register as a sex offender but that did not stop him from getting a South Carolina estate agent’s licence in 2006 and building a firm.

Sheriff Wright found it “strange” that Kohlhepp managed the pretext of a normal life for so long.

After the alleged link to the Superbike Motorsports massacre was revealed, Scott Ponder’s widow Melissa told reporters she had resigned herself to her husband’s murder never being solved, until she was phoned on Saturday evening by one of the case’s original detectives.

She told reporters that detectives had told her Kohlhepp was an angry customer who had been in the shop several times.

“He knew too much about the crime scene,” Ms Ponder said of Kohlhepp’s account to detectives. “He knew everything.”

Scott Waldrop, who has lived next door to Kohlhepp’s land for nearly 22 years, said he thought his neighbour was a serious Doomsday “prepper” who liked his privacy, but “he didn't seem like a threat”.

Mr Waldrop said when he saw the container, 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 up 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,” Mr Waldrop said.

Kohlhepp also has a house about nine miles away from his rural land in Moore, where neighbour Ron Owen said he was very private, but when they did talk across the fence, he was a “big bragger”.