Three people died and two are in critical condition after what officials are calling a carbon monoxide leak at a country music festival in Brooklyn, Michigan, NBC News reports.

Five people in their early 20s were found unresponsive in a travel trailer at a campground near the Faster Horses Festival Saturday afternoon, a statement from the Lenawee County Sheriff’s Department reads. Authorities became aware of the scene when a friend of the five men called 911 after not hearing from them.

While deputies from the sheriff’s department administered CPR after finding the men, three of them died on site, according to NBC News. The other two men remain in critical condition, as they are now reportedly being treated for acute carbon monoxide poisoning. 

Authorities are investigating it as a leak in relation to a nearby generator. The three-day “country music and camping festival” featured performances from some of the genre’s bigger stars, including Jason Aldean and Luke Combs.

Authorities are reportedly also investigating the death of a 30-year-old woman at the event, which is said to be unrelated to the other investigation

“First Responders stress the importance of keeping generators away from camping areas, tents, travel trailers, etc. as well as exhaust fumes from running vehicles,” the release read.