Thousands of troops have been vetted after they arrived in Washington DC ahead of Joe Biden's inauguration on Wednesday.

US defence officials said they are worried about an insider attack or other threat from service members involved in securing President-elect Joe Biden's inauguration, prompting the FBI to vet all of the 25,000 National Guard troops coming into Washington for the event.

The massive undertaking reflects the extraordinary security concerns that have gripped Washington following the deadly January 6 insurrection at the US Capitol by pro-Trump rioters.

READ MORE: Joe Biden Inauguration: What time does it start? Who is performing and does Trump have to attend?

And it underscores fears that some of the very people assigned to protect the city over the next several days could present a threat to the incoming president and other VIPs in attendance.

Army Secretary Ryan McCarthy said officials are conscious of the potential threat, and he warned commanders to be on the lookout for any problems within their ranks as the inauguration approaches.

So far, however, he and other leaders say they have seen no evidence of any threats, and officials said the vetting had not flagged any issues that they were aware of.

"We're continually going through the process, and taking second, third looks at every one of the individuals assigned to this operation," Mr McCarthy said in an interview after he and other military leaders went through an exhaustive, three-hour security drill in preparation for Wednesday's inauguration.

He said Guard members are also getting training on how to identify potential insider threats.

About 25,000 members of the National Guard are streaming into Washington from across the country - at least two and a half times the number for previous inaugurals.

And while the military routinely reviews service members for extremist connections, the FBI screening is in addition to any previous monitoring.

Multiple officials said the process began as the first Guard troops began deploying to DC more than a week ago.

And they said it is slated to be complete by Wednesday.

Several officials discussed military planning on condition of anonymity.

"The question is, is that all of them? Are there others?" said Mr McCarthy.

"We need to be conscious of it and we need to put all of the mechanisms in place to thoroughly vet these men and women who would support any operations like this."

In a situation like this one, FBI vetting would involve running peoples' names through databases and watchlists maintained by the bureau to see if anything alarming comes up.

READ MORE: Trump impeachment: Democrats build case alleging 'dangerous crime'

That could include involvement in prior investigations or terrorism-related concerns, said David Gomez, a former FBI national security supervisor in Seattle.
Insider threats have been a persistent law enforcement priority in the years after the September 11 2001, attacks.

But in most cases, the threats are from homegrown insurgents radicalised by al Qaida, the Islamic State group or similar groups.

In contrast, the threats against Mr Biden's inauguration have been fuelled by supporters of President Donald Trump, far-right militants, white supremacists and other radical groups.

Many believe Mr Trump's baseless accusations that the election was stolen from him, a claim that has been refuted by many courts, the Justice Department and Republican officials in key battleground states.

The insurrection at the Capitol began after Mr Trump made incendiary remarks at the January 6 rally.

According to Mr McCarthy, service members from across the military were at that rally, but it is not clear how many were there or who may have participated in the breach at the Capitol.

So far only a couple of current active-duty or National Guard members have been arrested in connection with the Capitol assault, which left five people dead.

The dead included a Capitol Police officer and a woman shot by police as she climbed through a window in a door near the House chamber.

General Daniel R. Hokanson, chief of the National Guard Bureau, has been meeting with Guard troops as they arrive in DC and as they gather in central Washington.

He said he believes there are good processes in place to identify any potential threats.

"If there's any indication that any of our soldiers or airmen are expressing things that are extremist views, it's either handed over to law enforcement or dealt with the chain of command immediately," he said.

The insider threat, however, was just one of the security concerns voiced by officials on Sunday, as dozens of military, National Guard, law enforcement and Washington, DC, officials and commanders went through a security rehearsal in northern Virginia.

The Secret Service is in charge of event security, but there is a wide variety of military and law enforcement personnel involved, ranging from the National Guard and the FBI to Washington's Metropolitan Police Department, US Capitol Police and US Park Police.

The bulk of the Guard members will be armed.