Carbon monoxide poisoning likely killed four people sleeping in a horse trailer at the World Clydesdale Show, Dane County Coroner John Stanley said Friday.

The two adults and two children were camping overnight outside at the Alliant Energy Center in the town of Madison. They were using a space heater to warm the trailer, Stanley said.

Stanley said relatives were being notified. The four came from central Wisconsin, he said.

The center is the site Friday through Sunday of the World Clydesdale Show. The show's Web site boasts, "The thundering hooves of over 500 Clydesdales are coming."

The carbon monoxide came from a portable gas heater that was not part of the trailer, said Dave Bloom, town of Madison fire chief. The bodies were discovered by other family members around 7:30 a.m. Friday.

They tried to wake up the four, but could not, Bloom said.

Stanley said there were two children and two older relatives who were not their parents. He did not know if they were the children's grandparents.

The children's parents were not at the show, he said.

The trailer was a combination camper and horse trailer, Bloom said. It was not equipped with a heater. There was one small vent in the roof that was open, he said.

However, carbon monoxide gas does not rise so having that open was of no benefit, he said.

Scott Gregory, town of Madison police chief, said there was "no indication of foul play whatsoever."

He said detectives were interviewing family members and hoped to confirm soon that the heater was to blame for the deaths.

Don Langille, who is with the Clydesdale Breeders USA, said the family members were longtime Clydesdale breeders and exhibitors who were well-known to everyone at the show.

It is common for people to sleep in a combination trailer. The horses were not in the trailer at the time.

Bloom said all four were sleeping in bunks.

"The majority of exhibitors will be saddened because they know everybody here, everybody grieves and they all grieve together," Langille said.