BTK (real name: Dennis Rader) tells TMZ…he was interviewed January 20 by Osage County Sheriff, Eddie Virden and three other investigators at El Dorado Correctional Facility in Kansas. That's where BTK is serving 10 life terms for killing 10 people from 1974 to 1991. He was captured in 2005 after sending police taunting letters using the moniker, "Bind, Torture, Kill."

Rader said the Osage County cops wanted to know if he was involved in the disappearance of 16-year-old Cynthia Dawn Kinney in Pawhuska, Oklahoma, on June 23, 1976. Kinney was reportedly last seen leaving Osage Laundromat and getting into a beige 1965 Plymouth Belvedere with two people under suspicious circumstances. No one has been arrested.

BTK said police thought he might have a connection to the Kinney case because he was actively committing murders in Wichita, Kansas, during that time frame. Wichita is only about a 2-hour drive to Pawhuska.

The killer also said cops asked him about his connection to Oklahoma and the Boy Scouts. BTK admitted he was in Oklahoma as a Boy Scout, but several years before and after 1976.

BTK categorically denied any involvement in the Kinney case.

Sheriff Virden confirmed to TMZ…he and the other investigators visited BTK in prison to interview the serial killer in a roundabout way regarding the Kinney case.

Virden said investigators were pursuing leads and doing their due diligence to see if BTK fits into the Kinney case considering his criminal activities around that time.

Virden said, “There’s similarities between the time frames. She disappeared in daytime – most of [BTK's] stuff occurred in daytime hours. Some of the stuff that motivated [BTK] -- there are similarities,” but Virden declined to further elaborate.

During the interview, Virden said he also gathered personal information from BTK about his early life, military service and his murders.