NORTH UTICA, Ill. (AP) — Three people died after apparently igniting a “black powder substance” and causing an explosion near a state park in northern Illinois, a coroner said.

First responders were called to an area along the Illinois River west of a Starved Rock State Park boat ramp where they they discovered three dead males around 7:20 p.m. Thursday, authorities said. Black powder was the original gunpowder and can be used to make fireworks.

“With assistance from the (Illinois) State Police, the Kane County Bomb Squad Unit & the FBI, it was determined that the individuals appeared to have ignited a type of black powder substance along an area near the riverbank,” LaSalle County Coroner Rich Ploch said.

The three died from injuries they received in the explosion, Ploch said. Autopsies were planned.

The victims’ names have not been released.

The explosion occurred about 75 miles (120 kilometers) southwest of Chicago.