Oklahoma authorities are working to recover more "trophies" that BTK serial killer Dennis Rader said he hid in multiple locations, the Osage County sheriff announced Tuesday.

"We have locations provided to us by BTK where he says he has other trophies hidden. We are in the process of working to go try to recover those items," Sheriff Eddie Virden said in a news conference.

The announcement comes less than a month after Osage County investigators discovered items in a "hiding hole" at the property of Rader's former family home. The items included "trophies" from at least one woman and bondage materials, authorities said. The search was based on a 2008 letter Rader wrote from prison describing "trophies" he had buried under the floor of his backyard shed.

Rader took and kept victims' personal effects, including jewelry and clothing, CNN has previously reported.

The sheriff, who announced an active investigation into potential links between Rader and cold cases in the region last month, said in Tuesday's news conference he interviewed the prolific serial killer Monday.

Rader was arrested and pleaded guilty in 2005 to 10 murders in the Wichita, Kansas, area that he committed in the 1970s through 1990s. He is known for his cat-and-mouse games with authorities, sending clues about his murders to law enforcement in the years before he was eventually arrested.

Now 78, Rader is serving 10 consecutive life sentences in a Kansas state prison.

In August, the Osage County Sheriff's Office publicly confirmed it was investigating Rader as the "prime suspect" in the 1976 disappearance of 16-year-old Cynthia Dawn Kinney and several other unsolved crimes in Oklahoma, Missouri and Kansas.

Authorities have said they believe the killer may have buried Kinney -- who was last seen at a laundromat in Oklahoma -- in a barn near the Kansas-Oklahoma border.

The sheriff confirmed during Tuesday's news conference that federal authorities have assisted the ongoing investigation, which was launched in January.

Virden also said investigators are in the process of verifying the claims of five people who have recently come forward saying they are surviving victims of Rader. Two of the five are from Oklahoma, the sheriff told reporters.

The sheriff's office has been poring through "very, very good tips" from the public regarding possible additional victims following CNN's exclusive reporting on Rader's detailed colored drawings of barns with female victims, which were recovered by law enforcement after his arrest in 2005.

The sheriff reiterated Tuesday the drawings could depict other crimes Rader committed.

"I can show you a drawing of an actual murder that matches the crime scene photo to a tee. So when I find large amounts of similar drawings in the same fashion, that makes me want to make sure that there's no other victims. And if there's anybody out there that can contradict that, I want them to come forward, because I don't want to waste my time," Virden said.

Virden also announced a National BTK Task Force Monday evening, naming investigatory experts, law enforcement and prosecuting agencies that will help in the investigation moving forward.

The task force includes the US Attorney's Office for the Northern District of Oklahoma and the Kansas Department of Corrections, which oversees Rader's imprisonment.

The Oklahoma State Bureau of Investigations also told CNN Monday it will be assisting the local investigators.