It has happened before:
The United States Capitol shooting incident of 1998 was an attack on July 24, 1998 which led to the death of two United States Capitol Police officers. Detective John Gibson and Officer Jacob Chestnut were killed when Russell Eugene Weston, Jr., entered the Capitol and opened fire.
Yes, and I especially remember that day because the owner of the business next door walked in and killed 2 of his employees and a court appointed attorney. A bunch of their people came running through my shop screaming for help, a single look at their faces told us that it was for real, every single guy here dove for their gun and faced the doors they ran in. Since that day every one of us has a gun squirreled around the place in our toolboxes or desk somewhere.
July 24 1998 was a day I won’t forget.