Riverside County sheriff's officials said Friday that they don't expect the 51-year-old woman to face criminal charges, as she is mentally ill.
Sgt. Kevin McDonald told The Press-Enterprise that the woman thought she was challenging the spirit of a dead motocross rider.
The threats were directed to a member of Metal Mulisha, Brian Deegan, who approached police about the bizarre comments in April.
"The comments were rambling and nonsensical but appeared to be from someone who felt they had been scorned by him," Investigator Brett Devine wrote in search warrant documents.
Investigators traced the comments to a home in Menifee, where they served a search warrant May 2, seized a computer and determined that the woman was mentally ill, authorities said.