Four people died overnight while camping outside the Alliant Energy Center in Madison. Investigators say they were overcome by carbon monoxide fumes. Dane County Coroner John Stanley says two adults and two children were found dead in a horse trailer where they had been camping for this weekend's World's Clydesdale Show at the Alliant Energy Center in Madison. The cause of the deaths appears to be carbon monoxide poisoning from a small space heater in the trailer. Stanley did not identify the victims or give their ages, but says the two adults are not the children' parents, and all four are from central Wisconsin.