What were the rumors about District 13 being burnt to dust (or rubble)?

In Part I, we learn that the Capitol led the other districts to believe that 13 was obliterated in the Dark Days, but actually, it was home to the Capitol's secret nuclear development program. However, rebels in 13 leveraged the substantial nuclear arsenal to threaten the Capitol and gain autonomy. Now, Katniss and all the other rebels and survivors live in District 13's underground community. Their lives are heavily regimented - individual schedules are imprinted on each person's arm every morning.