The FBI has warned of armed protests being planned for Washington and all 50 US state capitals in the run-up to President-elect Joe Biden's January 20 inauguration.
Threatened with more violence from outgoing President Donald Trump's supporters following last Wednesday's storming of the US Capitol, the FBI issued warnings for next weekend that run at least until Inauguration Day, the source said.
In other steps to safeguard the US capital, the National Guard was authorised to send up to 15,000 troops to Washington, and tourists were barred from visiting the Washington Monument until January 24.
The chief of the National Guard Bureau, General Daniel Hokanson, told reporters he expected about 10,000 troops in Washington by Saturday to help provide security, logistics and communications.
He said the number could rise to 15,000 if requested by local authorities.
At least one lawmaker asked the Pentagon to do more.
Senator Chris Murphy, who said he was sending a letter to the acting secretary of defence on Monday, said it was unclear if the National Guard would be sufficient to protect the nation's capital and that active-duty troops may be needed also.
"I'm not afraid of taking the oath outside," Biden told reporters in Newark, Delaware, referring to the traditional setting for the swearing-in ceremony on the Capitol grounds. But he said it was critically important that people "who engaged in sedition and threatened people's lives, defaced public property, caused great damage" be held accountable.
Biden's inaugural committee said on Monday the theme of the January 20 ceremony would be "America United".
Trump, who has sought unsuccessfully to overturn the November 3 election results with false claims of widespread fraud, said last week he would not attend the ceremony, a decision the president-elect supported.