Security has been ramped up at the US Capitol in response to “a possible plot to breach” the building.
The move was prompted by intelligence that an unnamed militia group planned the attack for 4 March – the day US presidents used to be inaugurated.
The House of Representatives cancelled Thursday’s session, but the Senate will continue with its agenda.
The threat comes two months after the deadly attack on the building by a mob loyal to former President Donald Trump.
They breached the Capitol building in January while lawmakers were inside moving to certify Democrat Joe Biden’s election victory. Mr Trump still refuses to admit losing the election.
The riot saw five people including a police officer killed and shook the foundations of American democracy. The head of the Capitol police force later resigned.
Supporters of an unfounded conspiracy theory believe Mr Trump will return to the White House on 4 March.
What does the intelligence say?
“As of late February, an unidentified group of militia violent extremists discussed plans to take control of the US Capitol and remove Democratic lawmakers on or about 4 March and discussed aspirational plans to persuade thousands to travel to Washington, DC, to participate,” a new intelligence bulletin issued by the FBI and Department of Homeland Security says.
Following that assessment, the US Capitol Police referred in a statement to “a possible plot to breach the Capitol by an identified militia group on Thursday, March 4”.
“We have already made significant security upgrades to include establishing a physical structure and increasing manpower to ensure the protection of Congress, the public and our police officers.
“Due to the sensitive nature of this information, we cannot provide additional details at this time.”
Why is Thursday significant for QAnon?
Supporters of an extremist conspiracy theory known as QAnon falsely believe Thursday will mark Mr Trump’s return to the White House for a second term.
They have latched onto this date because before the 20th amendment of the US Constitution – adopted in 1933 – moved the swearing-in dates of the president and Congress to January, American leaders took office on 4 March.
QAnon is a wide-ranging and completely unfounded theory which says that President Trump is waging a secret war against elite Satan-worshipping paedophiles in government, business and the media.
Security services were aware of online QAnon discussions surrounding 4 March, but did not “have any indication of violence or a specific, credible plot at this time”, an FBI official had told The Washington Post last week.
Capitol riot aftermath
The US justice department has charged more than 300 people with participation in the 6 January attack. Those arrested include members of the right-wing militia groups the Oath Keepers and the Three Percenters.
Democrats said the attack amounted to an insurrection and the House voted to impeach Mr Trump for allegedly inciting the mob. The former president – the first in US history to be impeached twice – was later acquitted in the upper chamber, the Senate, with many Republican senators sticking by him.
In late February Acting Capitol Police Chief Yogananda Pittman told Congress that the Trump supporters behind the January attack want to “blow up” the Capitol and kill lawmakers.
Leon Panetta, a former US Defence Secretary and CIA chief, told the BBC that police would take no chances following January’s attack.
“We have got to have constant intelligence on domestic terrorists, have to track their possible efforts to again repeat what happened on January sixth and I think that’s what you’re seeing now, is an abundance of caution to make sure that we are properly prepared to react, if in fact any group attempts any kind of armed attack again on the United States Capitol,” he said.