21.   NTSB investigators will try to find out whether another ignition source could have set off the explosion inside the tank.

22.   NTSB investigators have been trying for months to identify potential electrical ignition sources.

23.   Other potential ignition sources could include a spark created by an electrical fault in a wire, lightning or other static discharge.

24.   Several possible ignition sources could have caused the vapors to explode, according to Bernard Leob, director of aviation safety for the NTSB.

25.   That approach is to eliminate all possible ignition sources and to reduce explosive vapors that build up in center fuel tanks.

26.   The aviation investigators have settled on a static spark as a likely culprit because all other identified mechanical ignition sources appear even less likely.

27.   The accumulating gas was later ignited by an ignition source, possibly the pilot light on the water heater.

28.   The Federal Aviation Administration has favored first trying to identify and eliminate the specific ignition source that may have caused the crash.

29.   The FAA approach has been to look at ways to eliminate ignition sources.

30.   The new rules include a Special Federal Aviation Regulation to minimize the potential for failures that could cause ignition sources in fuel tanks on new and existing planes.