11.   Even a low-level ignition source, like an electrical arc, could cause the metallized Mylar insulation to catch fire, the FAA found.

12.   He said the system is designed to be free of ignition sources.

13.   If it found an ignition source, the consequences could be worse.

14.   In the meantime, the industry and the FAA say that they already are taking steps to make fuel tanks safer and to eliminate potential ignition sources.

15.   Investigators anticipate at certain times in the flight the mixture of fuel and air in the tank would sustain an explosion if there were an ignition source.

16.   Investigators for the National Transportation Safety Board have been trying for months to identify the electrical ignition source that caused the center fuel tank to blow up.

17.   Investigators have identified several potential ignition sources that are missing, like the probes that measured the amount of fuel in the center tank.

18.   Investigators have several theories but no evidence for a mechanical ignition source.

19.   Mix the fuel and the oxygen first, then introduce the ignition source, and you have a premixed flame.

20.   No ignition source was found.