Spellman was reported missing by her family last weekend, sheriff's Lt. Dennis O'Leary said. She was last seen leaving her mother's house where she lived.
A passerby spotted her body in a field in the area of Fourth Street West and West McArthur Street, the sheriff's office said.
Spellman had mental health issues and foul play is not suspected, O'Leary said.
Her body was identified by her family and through fingerprints, O'Leary said.
There were no signs of trauma on Spellman's body and authorities suspect she might have overdosed on a drug, O'Leary said.
Sheriff's officials said the body had been there for about a week and was hidden by tall grass.