The US House of Representatives has cancelled its planned session today amid fears of unrest at the Capitol Building.
Police have warned that a militia group could be planning an attack.
The building has been surrounded by high fencing and National Guard troops ever since it was stormed by supporters of former president Donald Trump on 6 January.
Right-wing conspiracy theorists have falsely claimed that Mr Trump will be returned to power today.
They see the date of 4 March as significant as it used to be the day when presidents were inaugurated.
Mr Trump was defeated in an election last November by Joe Biden, who was sworn in as US President on 20 January.
Police say they are taking the threats of violence seriously and the House of Representatives has cancelled its planned session.
No further details on the threat were provided.
The Capitol Police statement noted that it already has made "significant security upgrades" at the Capitol, home to both the House of Representatives and Senate.
It was unclear if these upgrades were done in response to this latest threat or whether it includes the measures already in place following the 6 January riot.