At least four people died while attending a weekend country music festival in southern Michigan, including three men who likely succumbed to carbon monoxide exposure inside a travel trailer, authorities said Saturday (July 17).

Two more men were in critical condition at a hospital from the same exposure, the Lenawee County sheriff’s office said. The five men were in their early 20s.

A concerned friend called 911 around 1:30 p.m. when he had not heard from them during the Faster Horses Festival at Michigan International Speedway, 80 miles (128.7 kilometers) west of Detroit. They were staying at a campground outside the festival grounds in Woodstock Township.

“This tragic incident is being investigated as a suspected carbon monoxide exposure from a generator located very near the travel trailer,” the sheriff’s office said on Twitter.

Separately, state police reported the death of a 30-year-old woman, Melissa Havens of Croswell.

The cause and manner was unknown, state police said, although investigators were looking for a suspect.

“Detectives want the public to know there is no danger or threats to people attending” the music festival, state police said.

The three-day festival is scheduled to conclude Sunday. Its headliners include Luke Combs, Thomas Rhett and Jason Aldean.