The 1-year-old died Jan. 6 after police and emergency responders were called to a home at 1801 Faraon on an unresponsive child call.
A preliminary autopsy determined the infant died from hyperthermia and showed no indications of direct physical trauma.
According to a St. Joseph Police Department investigation, the infant was placed in a portable playpen with an additional blanket after a power outage. The playpen was wrapped on three sides by a comforter to hold in heat and placed close to a heating vent.
Police say adults in the residence found the infant unresponsive around six hours after power was restored.
Buchanan County prosecuting attorney Dwight Scroggins said no crime was committed and ruled the death was accidental.
"The case is a tragic example of well-intended but unknowingly dangerous steps to protect the child being taken, which resulted in the unforeseeable accidental death of the infant," Scroggins said.
Scroggins' office received the case after detectives had finished their investigation.
Hyperthermia, is a heat-related illness brought on by an elevated body temperature.