A Dallas apartment building partially collapsed Wednesday after residents reported a carbon monoxide leak, according to KVIA. Firefighters arrived to the building around 10:30 a.m. after residents reported the smell of gas, but as they investigated further, an explosion occurred and led to half of the building collapsing. Authorities said seven people, including four firefighters, were injured, with two people in critical condition. “The damage is extensive,” Dallas Fire-Rescue spokesman Jason Evans said. “The collapse itself is extensive.” It is unclear what caused the explosion. Residents reported smelling gas as early as Tuesday night.