1.   The blasts were caused by pockets of methane gas that ignited.

2.   No one knew exactly what caused the blast.

3.   A gas leak is believed to have caused the blast.

4.   A gas leak was believed to have caused the blast.

5.   A simple metal pin was used instead of a padlock and the valve was, in fact, opened, creating the conditions that caused the blast moments later.

6.   Explosions sounded on the Taliban side, but it was unclear whether U.S. bombs or alliance shells caused the blasts.

7.   However, an accumulation of rainwater may have caused the blast, said Battalion Chief Steve Borgogno.

8.   Investigators have known for months that the center fuel tank exploded but they have not been able to determine what caused the blast.

9.   Investigators have said the deadly blasts were caused by an explosive that resembles sugar and authorities have warned citizens to look out for anyone carrying large bags into basements.

10.   Lajous said the blast was probably caused by poor execution of standard maintenance procedures.