Fire officials referred to the incident as a “dairy accident,” but did not provide additional details. A preliminary investigation suggests the six victims were exposed to gas at Prospect Valley Dairy near Keenesburg, a northern Colorado town about 40 miles outside of Denver, the Weld County Coroner’s Office told CBS News.

Fire crews were dispatched to a dairy farm in Weld County on Wednesday evening for a "confined space rescue," the Southeast Weld Fire District said.