UPDATED: Franklin County Sheriff Gary Toelke said an autopsy conducted Tuesday morning did not reveal the cause of death of a young boy who collapsed Monday afternoon on a school bus.

Toelke said investigators are waiting for results of toxicology testing. That could take several weeks to complete, he said. 

The sheriff said everyone involved in the investigation are working together to determine the cause of the boy's death. No foul play is suspected.

Bradley Voss, 8, a student at the Strain-Japan School in southwestern Franklin County, complained of a headache Monday morning before leaving for school and was given some standard pain medication.

School officials said the boy did not show any signs of illness during the day and after school he boarded his bus for home. While on the bus, he fell to the floor and was unresponsive, according to the report.

The bus driver administered CPR until emergency crews arrived and transported the boy to Missouri Baptist Hospital in Sullivan where he was pronounced dead, the sheriff said.