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.

Layout
The Capitol Building complex comprises four areas, two of which can be directly accessed from the Capital Wasteland. The "Capitol Building" entrance from the west, in The Mall, exits into the Capitol Building west entrance area. The Seward Square entrance from the east is one of two doors into the Capitol Building west entrance.

Upon entering from the west, the other door into the Capitol Building east entrance is immediately ahead and to the right. The entrance to the third area, the Hall of Columns, is in almost exactly the same direction, east-southeast, but about six times further away, past the super mutant behemoth in the rotunda. At the end of the Hall of Columns are two stairwells; the door at the bottom of the second accesses the fourth area, the Conference Room.

A door in the northwest corner of Conference Room area leads back to a room one floor above the southeast corner of the west entrance area, but it is effectively a one-way passageway, as when the player goes through, the floor of the upstairs room caves in and crashes down to the room beneath. This also triggers a second battle between opposing forces in the rotunda (Talon Company and mutants, although these battles may differ by level).

Rotunda
Inside the Capital Rotunda, the Lone Wanderer will initiate a battle between the Talon Company and the super mutants. In the center of the rotunda is a super mutant behemoth. Several other mutants are also present, ranging from basic all the way to master. Two super mutant overlords are also present if the player has installed Broken Steel.

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

 * A Tales of a Junktown Jerky Vendor is on a desk in a row of four desks separated by screens; it is the one with the fallen filing cabinet. The row of desks is just north of the long hallway running east and west to either end of the area.
 * A pre-War book is located on the fallen cabinet next to the desk with the Tales of a Junktown Jerky Vendor on it.
 * Two stimpaks and a Nuka-Cola Quantum are found in a break room east of the desks with the pre-War book and the Tales of a Junktown Jerky Vendor.
 * A Lying, Congressional Style can be found south of the Capitol Building east entrance at Seward Square, in a Pulowski Preservation shelter.

Conference Hall

 * Lying, Congressional Style appropriately stashed on a podium.

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 on the west side.

Behind the scenes
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

Bugs

 * The super mutant behemoth can sometimes spawn inside the small room with the turret control computer next to the rotunda.
 * Although it may well be intended by the developers to prevent players using the behemoth's size to their advantage, the super mutant behemoth cannot be damaged from outside the rotunda room.
 * The behemoth can sometimes spawn dead, with the Talon Company mercs shooting at its dead body.