A training exercise accident left 22 people injured at the Marine Corps Air Ground Combat Center at Twentynine Palms, Calif. on Thursday. Officials said an equipment malfunction caused an extinguishing system to set off inside an assualt vehicle during a drill, exposing 22 Marines to the fire retardant gas halon. There was no fire or explosion and all 22 were in stable condition, according to base spokesman Dave Marks. Three Marines were kept overnight and the rest were allowed to continue training. Halon is used in fire extinguisher systems and can cause breathing problems from exposure to high concentrations. The Marines were from the 2nd Battalion, 3rd Marine Regiment, based in Hawaii.