The Capitol Building

Before the Great War, the United States Capitol Building housed the legislative branch of the government of the United States. It is located in Washington, D.C., on top of Capitol Hill at the east end of the Capital Mall. The other side of the Capitol opens onto Seward Square.

Sections
The building is broken into a number of areas. The most common way to enter the building is from the west side of the building from the mall, and this takes you to the appropriately-named "Capitol Building West Entrance". One can also explore the "Capitol Building East Entrance", the "Conference Room", and the "Hall of Columns". The Rotunda, although centrally located, is considered part of the "Capitol Building West Entrance".

The Battle for the Capitol Building
The Capitol serves as the battleground for a major conflict between the Super mutants, Talon Company and the Enclave , although its not unheard of for there to be slavers, raiders, or Brotherhood of Steel Knights there as well.

An Enclave Vertibird and kill team (with an Enclave-controlled Deathclaw) may be encountered on the steps of the Mall side of the building, while the rotunda of the building hosts a Super Mutant Behemoth (see Super Mutant Behemoth below). Not only are there super mutants near the rotunda of the building, there are also super mutants right before you go up the steps to the building.

Super Mutant Behemoth
One of the five Super Mutant Behemoths can be found in the Capitol Rotunda.

Capitol Building West Entrance

 * Nikola Tesla and You on a desk behind a [hard] locked door.
 * U.S. Army: 30 Handy Flamethrower Recipes on a table, southeast part of the Capitol dome
 * Mini Nukes and a Fat Man on opponents and/or on shelves inside the dome

Capitol Building East Entrance

 * Tales of a Junktown Jerky Vendor on the middle desk in a row of three cubicles
 * Pre-War Book on a fallen cabinet next to that desk
 * Nuka-Cola Quantum a room over to the east, behind a coffee maker

Conference Hall

 * Lying, Congressional Style appropriately stashed on a podium.
 * A bottle of Nuka-Cola Quantum can be found on the South side of the second floor in the Nuka-Cola Vending Machine laying flat.

Hall of Columns

 * D.C. Journal of Internal Medicine on the upper level. A Talon Company sniper is stationed on a bridge, and the book is on a railing facing south. If it is not there it was knocked down in the fire fight (this is common) just reload or look for it on the floor below.
 * Chinese Army: Special Ops Training Manual behind a {hard} locked door (accessed from a small hallway found near a turret and a locked turret control terminal in the middle of the hall)

Gallery and concept art
On May 8, 2007, Bethesda Softworks released a piece of Fallout 3 concept art by Craig Mullins depicting the Capitol after the Great War. This picture appeared then in an apparent terrorist threat, said to be shown in some islamist forum as a real-life would-be scenario, according to media. Source: http://www.news.com.au/al-qaeda-urges-use-of-wmd/story-e6frfkp9-1111116473744