The Capitol Building

Before the Great War, the United States Capitol 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.

The Capitol serves as the battleground for a major conflict between the Super Mutants and the Talon Company. An Enclave Vertibird and strike group may be encountered on the steps of the Mall side of the building, while the dome of the building hosts a Super Mutant Behemoth.

The building is broken into a number of areas. The most common way to enter the building in Fallout 3 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 dome, although centrally located, is considered part of the Capitol Building West Entrance.

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

Captiol Building East Entrance

 * Tales of a Junktown Jerky Vendor on a desk in an office
 * 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.

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)
 * 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)

Super Mutant Behemoth
The Behemoth can be found in the Central Dome of the Building. There are two ways to go about killing him.

If you just want him dead, the smartest move would be to enter the hallway next to the entrance to the dome area, this should lead you to a terminal you can hack to activate the turret system. Remember to deactivate the turrets, as they will be hostile to everything in the room. After he is taken down, you can loot the various Mercs and Mutants for a large amount of ammo. There is also a large cache of missiles you can access once the room is clear.

If you wish to take him down yourself for experience, be wary of the Mercs. One comes equipped with a Fatman, so it would be wise to enter sneaking mode and quickly take him out. At that point, he should go down with one or two mini nukes, or a few missiles. There is also a Super Mutant with a minigun on a ledge in the back of the room. If you are low on ammo, there is a surprising amount on the corpse of the Behemoth.

When i got in there ready to face the Behemoth, i found him to already be dead. I dont know if that was because of some glitch, or maybe the Talon Company managed to take him out, but one of them did have a fatman, so it's possible. Stood near the scene was a non agressive Talon Merc, with a fatman.

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, in a largely intact state.