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.
It came amid preparations for a rehearsal for Mr Biden’s inauguration.
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 after Monday’s lockdown. Congress is currently in recess.
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.
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.
What will happen on 20 January?
Mr Biden and Vice-President-elect Kamala Harris will take the oath of office in front of the Capitol building, overlooking the National Mall.
Normally a crowd of hundreds of thousands would be there to witness a significant moment in American national life.
However, between the coronavirus pandemic and security concerns, the size of the crowd is being severely restricted this year.
Once sworn in, Mr Biden will give an inaugural address, setting the agenda for his term in office.
The ceremonies include a poetry reading, the Pledge of Allegiance and a benediction. Lady Gaga will sing the national anthem, and there will be a musical performance by Jennifer Lopez.
One prominent figure will be conspicuous by his absence: Mr Trump has said he will not attend the inauguration.
On Monday, outgoing First Lady Melania Trump released a farewell message asking Americans to “lead by example” in caring for others.
Normally the first lady hands over to her successor by inviting her for a walk-through of the private quarters in the White House.
However, Mrs Trump is set to defy tradition, snubbing the president-elect’s wife, Jill Biden, CNN reports.
Tom Hanks will host an event on prime-time TV on the night of the 20th, with performances by Justin Timberlake and Bon Jovi.
Mr Biden’s inauguration planning team has designated the day before the swearing-in as a nationwide memorial to lives lost through Covid-19.