A radio recording released Friday indicates that six minutes elapsed between the time emergency tones were sounded to announce the creation of a collapse zone at a fire Oct. 12 and when a wall collapsed, killing two Kansas City firefighters. The timing is significant because safety experts say all fire personnel should be instructed to exit a collapse zone immediately after one has been established around an empty building. The only exception is to save lives, but never to save property, according to the National Institute of Occupational Safety and Health, which investigates firefighter deaths. The exact timing could not be established previously. An edited copy of recorded radio traffic that was released to the news media several days after the fire made that impossible. It compressed the timing by editing out dead air between transmissions by firefighters on the scene and emergency dispatchers. That recording indicated that the two firefighters who died and two who were injured might have had as little as 2 1/2 minutes to exit the alley next to a burning building in the 2600 block of Independence Boulevard. Fire Chief Paul Berardi told The Star this month that it is common practice at his department to edit out the dead air as a matter of convenience when radio recordings are released to the public.