When she died, Sheriff Gene Kelly said Willison was on medication and dealing with mental issues.
"She had left a note at her house. She indicated that she was going to return to mental health," Kelly said.
However, the next morning, a delivery driver saw Willison's car crashed into a tree, but by the time he turned around, the driver was gone.
Authorities searched the area, but found no trace of her. Kelly said he speculated that Willison may have been disoriented and wandered farther north than they anticipated.
"With the rain that we have had this year, it is possible that it is what caused her to be washed into the area where her skull was found," Kelly said.
News Center 7 talked with Willison's sister, Christi Moore, who lives in Colorado and she believes foul play was involved.
"I think she ran into foul play. I don't know who would have hurt such a sweet person," Moore said.
However, Kelly said there is no indication that Willison was killed.
"Our Clark County coroner examined this skull and sees no physical signs of trauma," Kelly said.
Willison was a mother and was well-liked in the community.
"She was very caring. She always thought about other people before herself," Moore said.
Kelly said there are hundreds of acres north of where the skull was found where Willison's remains could be. Without help, he said it could be a long time before her body is found.