The acting Pentagon chief said on Monday the FBI is assisting the US military in vetting more than 25,000 National Guard troops being deployed to assist in protecting the US Capitol around President-elect Joe Biden's inauguration for potential security concerns. 

After the Jan 6 Capitol assault by supporters of Donald Trump that resulted in five deaths and sent lawmakers into hiding, the US government has imposed unprecedented security surrounding the Capitol, including non-scalable fences rimmed with razor wire and a large security zone that the public is barred from.

Acting Secretary of Defense Christopher Miller said in a statement on Monday the vetting is "normal for military support to large security events... While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital."

The Washington Post reported Monday the FBI in an intelligence report warned law enforcement agencies that far-right extremists had discussed posing as National Guard members in Washington.

Asked why authorities were looking into the background of every National Guard member called in to help secure the area around the swearing in ceremony, Acting Department of Homeland Security Secretary Peter Gaynor said on Monday, "The FBI and others decided this would be a prudent move."

Meanwhile, a young woman identified as having taken part in the storming of the US Capitol reportedly stole a laptop from top Democrat Nancy Pelosi's office and hoped to sell it to a Russian spy agency, according to an FBI criminal complaint.

The Washington Post reported Monday night that federal authorities said they had arrested Riley June Williams.