Todd Kohlhepp, who is serving a life sentence for murdering seven people, said he tried to tell investigators about other crimes he had committed - but was "blown off" by the FBI and is no longer willing to co-operate.
In an eight-page letter to the Herald-Journal of Spartanburg, the 46-year-old wrote: "At this point, I really don't see reason to give numbers or locations."
Kohlhepp, from North Carolina, was arrested in November 2016 after Kala Brown was found chained to a wall inside a metal shipping container kept in his garden. She had been raped almost daily.
The 30-year-old and her boyfriend, Charles David Carver, had gone missing at the end of August 2016 after they went to clean his home.
Kohlhepp shot Mr Carver multiple times then took over his Facebook account, sparking suspicion.
Two further bodies were discovered buried on his property and he then confessed to fatally shooting four staff members at a motorcycle shop in 2003 after they "made him angry".
In the gun obsessive's home, and another property he owned, detectives found an "arsenal" of weapons after finding Ms Brown and the three bodies.
Ms Brown told the Dr Phil television programme that Kohlhepp told her he had more victims and bragged he was "nearing the triple digits".
Don Wood, chief division counsel at the FBI's Columbia office, said there is a pending investigation but would not comment on specifics.
In May this year, Kohlhepp was sentenced to life in prison after admitting to killing seven people.
When he was 15 years old, he was jailed for 15 years for kidnapping and raping a 14-year-old girl - and he was registered as a sex offender.