You are also correct that it probably wasn't Martial Law because DC is the Federal City and not a State subject to civil law.
DC is absolutely subject to civil law.
True, and the DC National Guard is subject to the Posse Comitatus Act, limiting the power of the military to enforce the local laws.
During the 1970's anti-war demonstrations, the DC Guard MP units augmented the DC Police in a law enforcement role. To avoid possible violations of that act, the Guard units were sworn in as DC Police, thus having the same law enforcement powers as the regular police.
They even gave us a real adhesive backed stick on DC Metropolitan Police badge to wear on our flack vests.
