The Capitol complex in Washington DC was briefly locked down after a security alert, two days before Joe Biden is inaugurated as US president.
Police say they acted out of an abundance of caution after witnesses reported smoke rising nearby. The fire was several blocks away.
A rehearsal for Mr Biden's inauguration was suspended.
Five people died on 6 January when a pro-Trump mob stormed the Capitol, which is home to the US Congress.
Security is tight, with thousands of National Guard members deployed at the Capitol and around central Washington DC.
An alert circulated earlier to Capitol staff said the complex was on lockdown, with no-one being allowed in or out.
But the Washington DC fire service said it had responded to a fire nearby, which was now extinguished.
There was no threat to the public, officials said.
All 50 US states and the District of Columbia (DC) are on alert for possible violent protests, ahead of Mr Biden's inauguration on Wednesday.
The FBI has warned of possible armed marches by pro-Trump supporters across state capitols.
Congress is currently in recess and will not reconvene until after the inauguration.