WATERLOO, Iowa (KWWL) -- Waterloo Fire Rescue confirmed crews were called the the Cattle Congress around 9:20 Saturday night.
Individuals displayed signs of carbon monoxide poisoning.
Three were people were transported to the ER, but authorities are unsure how many people may have taken themselves to the hospital.
Their conditions currently are unknown, as is the cause for what started the incident.
The investigation is ongoing.
However, KWWL spoke to a witness who attended the Monster Track show, and they believed there were ventilation issues in the Hippodrome.
Stay with KWWL as we continue to learn more about this developing story.