A Green Valley man was killed in an ATV accident Saturday morning southwest of Tucson.

Terry Duncan, 68, was riding an all-terrain vehicle in a desert area about 10 miles southwest of Green Valley at about 11:40 a.m. when he drove down a steep hill, fell off the ATV and hit his head on the ground, according to a news release from the Pima County Sheriff’s Department.

A friend riding another ATV tried to revive Duncan, but he was pronounced dead at the scene.

Duncan was not wearing a helmet.