Wisconsin ? Carbon monoxide from an old portable propane heater killed two children and their grandparents as they slept in a combination camper-horse trailer ahead of a horse show, authorities said Friday.

The four were discovered by other family members who came to wake them on Friday morning around 7:30 a.m., just as the World Clydesdale show was starting at the adjacent Alliant Energy Center.

Dane County Coroner John Stanley identified the victims as DeVere Clay, 68, his wife Barbara, 57, and grandchildren Erin Briney, 10, and Hope Briney, 13.

Don Langille, a spokesman for Clydesdale Breeders of the U.S.A., said the victims were longtime Clydesdale breeders and exhibitors who were well-known to everyone at the show.