The Osage County sheriff says serial Killer Dennis Rader, known as "BTK," is the prime suspect in the disappearance of a Pawhuska woman in 1976.

Sheriff Eddie Virden says they took another look at the case of Cynthia Kinney, and that led them to search Rader's former Kansas property in April and this week.

Virden says they interviewed Rader in prison in Kansas in February but didn't ask him about Cynthia Kinney; they just wanted to see if he had any ties to Osage County. But in that interview, Rader told investigators he had a fantasy of kidnapping a woman from a laundromat.

Sheriff Virden says there are many circumstantial pieces of evidence that tie Rader to the disappearance of Kinney. Rader was an active serial killer when she disappeared. He lived two and a half hours away, most of his murders were during the week from eight to noon, and Rader worked around this region. Virden says Kinney disappeared in the middle of the week, in the morning in 1976.

Investigators searched Rader's former property in Park City, Kansas, because Rader said in a letter that he buried the driver's licenses of his victims behind his house. Virden says they found a very old pair of pantyhose when they searched in April, which they believe were used to tie someone up. They went back and searched under a sidewalk this week and found more items of evidence.

"Our hopes are, knowing that he kept trophies and different clothing articles and things from victims, that some of our victim's items or other unknown victim's items would be in that evidence, and if it is, we would hope that it could get processed and checked for DNA for unknown or other victims of cold cases,” said Sheriff Virden. "We don't want to bring more pain to the family. We'd like to bring answers, or closure, and justice for the victim."

Sheriff Virden says they still have a lot to do, like more interviews and work with other law enforcement agencies. He says they want to find the truth for the victim's family.

Related: Dennis Rader Known As 'BTK' Considered Suspect In Pawhuska Woman's Disappearance, OCSO Says

BTK serial killer suspect Dennis Rader pleaded guilty in 2005 to 10 counts of murder, including that of Delores Davis, a 62-year-old woman who was abducted from her home in 1991 and found nearly two weeks later strangled under a bridge.

Rader, 60, of Park City, had been accused of killing 10 people in the Wichita area between 1974 and 1991. Davis' murder was the last one attributed to the BTK Killer.

There was no plea deal.

BTK stands for "Bind, Torture, Kill." In messages to the press and police, the serial killer used the initials to describe how he treated his victims: Bind them, torture them, kill them.

BTK sent letters to media about the crimes in the 1970s, but stopped for more than two decades before re-establishing contact in March of 2004 with a letter about an unsolved 1986 killing.

Since then, authorities said the killer had sent at least eight letters to the media or police, including three packages containing jewelry that police believed may have been taken from BTK's victims. One letter contained the driver's license of victim Nancy Fox.

CBS News Correspondent Cynthia Bowers reported the killer teased police with cryptic clues that may be true or not: that he was born in 1939, raised by a widowed mother and is obsessed with railroads. A week before the arrest, three packages were sent to a local TV station containing jewelry that could have been from a victim.

The letters sent chills through Wichita, but also rekindled hope that modern forensic science could find some clue that would finally lead police to a killer most thought was dead or safely locked in prison for some other crime.

Thousands of tips poured in, and the Kansas Bureau of Investigation conducted hundreds of DNA swabs in connection with the BTK investigation.

A source with knowledge of the investigation who spoke on condition of anonymity said surveillance gave police their "first big piece" of recent evidence, leading authorities to a vehicle and the suspect.

One of the victims newly identified by police, 53-year-old Marine Hedge, lived on Rader's street in Park City. She was abducted from her home in 1985, and her body was found eight days later along a dirt road.

"This has not been an easy task," Wichita Mayor Carlos Mayans said. "Our fine police department has been, at times, questioned. Their competence was questioned, and their actions were often second-guessed. But all the while, these officers were steadfast in their commitment to solve the biggest police case in Wichita's history."

The BTK slayings began in 1974 with the strangulations of Joseph Otero, 38, his wife, Julie, 34, and their two children.

The letters began that same year, with poems and graphic descriptions of the crimes. The killer even called police with details of Nancy Fox's 1977 slaying.