(NewsNation) — The Osage County Sheriff’s Office in Oklahoma is looking into missing persons cases possibly associated with Dennis Rader, the serial killer dubbed BTK.

The cases include that of 22-year-old Shawna Beth Garber, whose body was discovered in 1990. An autopsy revealed she had been raped, strangled and restrained with bindings about two months before her body was found in McDonald County, Missouri.

Missouri detective Lorie Howard has spent hours speaking with Rader and said she’s always found him to be “very cooperative.”

“If I offered up something, he (Rader) gave me an answer. He never held anything back. He actively participated for the whole interview,” Howard said during a “Banfield” appearance Thursday night.

As part of the investigation into Rader, authorities conducted a dig this week near his former Kansas property in Park City, finding “items of interest” including tangled pantyhose.

Lead investigator Osage County Sheriff Eddie Virden showed NewsNation the pantyhose and discussed the other items.

“You can still see where they’re knotted, where they appear to have been around. Probably somebody’s wrist,” Virden said. They also found jewelry and broken glass, among other things.

The investigation into whether Rader was responsible for additional crimes started with the reexamination of the disappearance of Cynthia Kinney, a 16-year-old cheerleader who was last seen at a laundromat in Pawhuska, Oklahoma.

The Associated Press contributed to this report.