The J. Edgar Hoover Building - FBI Headquarters
This was an "attraction" recommended to me by a friend who visited DC last year. I hadn't realised he'd also been there over 8 years ago too and that's when he'd done the FBI Tour that he recommended.
The place is in total lockdown post 9/11. Great big immovable shipping containers block the former entrances.
However it's a great example of Brutalist architecture.