Capitol Police spokeswoman Kimberly Schneider says the suspected shooter was "neutralized" after sustaining a self-inflicted gunshot wound. It's unclear whether the man is dead. No one else is believed to be hurt.
Schneider also says Capitol Police are investigating a suspicious package on the lower west terrace of the building.
No one is being allowed to enter or exit the Capitol and the visitors' center, and some streets around the complex are closed.
The lockdown comes during Washington's annual Cherry Blossom Festival, which attracts many tourists.