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 after a pro-Trump mob stormed the Capitol, which is home to the US Congress, on 6 January.
Thousands of National Guard reserve soldiers have been deployed at the Capitol and around central Washington DC.
There was no threat to the public, officials said. Congress is currently in recess and will not reconvene until after Wednesday’s swearing-in ceremony.
What preparations are taking place for Biden’s inauguration?
Security is tight after the rioters overran the Capitol earlier this month.
The National Mall – the landscaped park around the complex – has been closed, along with many major roads. Fences have been put up around the White House.
The inauguration rehearsal scheduled for Monday has already been postponed once on security grounds.
Because of the coronavirus, the size of the crowd on 20 January will be severely limited. Only 1,000 tickets will be made available, instead of the usual 200,000.
All 50 US states and the District of Columbia (DC) are on alert for possible violent protests.
The FBI has warned of possible armed marches by pro-Trump supporters across state capitols.
Once he is sworn in, Mr Biden will issue executive orders to reverse President Trump’s travel bans and re-join the Paris climate accord.
The president-elect is also expected to focus on reuniting families separated at the US-Mexico border, and to issue mandates on Covid-19 and mask-wearing.