National Records Fire
In the summer of 1973, 16 to 18 million military records and documents were lost in the National Personnel Record Center (NPRC) fire. Although FBI investigators found cigarette butts on the sixth floor of the center, where the fire was isolated, it is believed that something else caused the inferno. The flames could have been deliberately started by terrorists. Still others believe that the government "attacked" itself in an effort to destroy classified files, some from WWII. Since none of the documents had duplicate copies, or had been indexed, it's hard to know what was lost in the destruction or why.