Combustion has four major stages: incipient, when the fire is beginning and temperatures are below 800 degrees; free burning, when materials in the room have reached the ignition point; flashover, when all room contents have reached the critical ignition point and there is sufficient oxygen to sustain the fire; and backdraft, when the fire smolders because of lack of oxygen, but may reignite with the introduction of additional oxygen. In deciding to enter a burning building, the police officer should first consider dangers from poor visibility, oxygen deficiencies, super-heated air, and smoke and gases. The officer also should observe the scene for people leaving the scene, barriers to fire suppression, wind direction and velocity, and signs of forced entry or unusual odors. Police officers entering a fire scene should make sure someone knows of their entry, check for backdraft conditions and visible fire signs, and check entry door for heat and pressure. Officers should enter in a low crawling position, remember directions, and control their breathing. During the fire investigation, police should determine the cause of the fire, its point of origin, who is responsible and advocate corrective action in the future. Photograph.