WICHITA, Kan. (KSNW) – Investigators named serial killer Dennis Rader, also known as BTK, as the number one suspect in an Oklahoma missing person’s case from the 1970s, 18 years after he was arrested.

New evidence led them to the site of Dennis Rader’s former home in Park City, where they dug into the ground where a shed once stood.

It’s all part of an effort to find out what happened to Cynthia Dawn Kinney, who was last seen at 16 years old leaving a Laundromat in Pawhuska, Oklahoma, in June of 1976.

The sheriff’s office started investigating the case again in December after new discoveries tied Rader to Kinney’s disappearance, according to them.

Nearly 50 years ago, investigators suspect Rader was installing security equipment at a bank in Pawhuska, Oklahoma.

Across the road was a laundromat, the last place anyone saw Cynthia Dawn Kinney.

That link is what led the Osage County Sheriff Eddie Virden to start looking at Rader as a suspect in her disappearance.

“She was taken mid-morning in the week,” Virden said. “A lot of his crimes occurred between eight and noon in the week when he worked for ADT.”

They discovered more evidence in a book that was being written by Rader before his arrest. There, he discussed a project he called “Bad Laundry Day,” according to Virden

“He would go to a laundromat in Wichita and watch victims and young women,” Virden said.

That passage was written in 1976, the same year Kinney disappeared.

“The further we went in, the more information we gained that made us believe he was the one who committed this crime,” Virden said.

Other writings from Rader described where he buried items belonging to victims: Under his storage shed.

In April, Osage County law enforcement dug up the site where the shed once stood and found “pantyhose ligature” they believe Rader used to tie up victims.

They returned this week, removing a section of sidewalk to uncover more evidence.

The sheriff’s office is not describing what they found Tuesday, but they are calling them “items of interest.”

They say they found them exactly where Rader described placing them.

Virden believes four other people from Missouri, Oklahoma, Hays and Wichita could also be Rader’s victims based on new evidence.

Rader maintains he has no involvement in Kinney’s disappearance.