Four men were found dead inside a trailer Saturday during a music festival in Michigan. Police believe it is a case of carbon monoxide exposure.

Two other men inside the trailer were taken to a hospital after being found at the scene unconscious.

The pair were in critical condition and are being treated for acute carbon monoxide poisoning, police said.

Emergency officials received a phone call around 1:27 a.m. Saturday about people not showing up at a camp in Woodstock Township during the Fastest Horses Festival, a multi-day country music festival.

"The caller was a friend of 6 people and was upset when he did not hear from them," police said.

Police are investigating the "tragic incident" as a suspected exposure to carbon monoxide from a generator located near the travel trailer.