A person shot him or herself dead near the United States Capitol building, sparking a security lockdown at the complex, US Capitol police said.
Police were also investigating a suspicious package on the lower west terrace of the Capitol building in Washington on Saturday, and several nearby streets were closed.
Several media reports said it was luggage left by the person.
Police said the lockdown was a precautionary measure.
"There are no indications at this point of terrorism," a US official told NBC News.
"The Capitol has been locked down because of the potential security threat. No one can enter or exit," Capitol Police spokeswoman Kimberly Schneider said in a statement.
"Confirmed: self-inflicted gunshot by neutralised subject," she added shortly afterwards. She did not say if the person was a man or a woman.
The incident occurred while Congress is out of session this week, so few lawmakers were in town. But the city was crowded with tourists visiting for the popular annual Cherry Blossom festival.