TEHRAN—The family, clad in black, stood at the curb of the road sobbing. A middle-aged mother slapped her cheeks, letting out piercing wails. The father, a frail man who worked as a doorman at a clinic in central Tehran, wept quietly with his head bowed.

Minutes before, an ambulance had arrived from Tehran's morgue carrying the body of their only son, 19-year-old Kaveh Alipour.

On...