Fears about a potential insider attack at Joe Biden’s inauguration have prompted the FBI to vet all 25,000 National Guard troops who will be on duty.
Officials are taking “second, third looks at every one of the individuals”, said US Army Secretary Ryan McCarthy.
Guard members are also getting training on how to identify potential insider threats, but Mr McCarthy said they had not found anything of concern so far.
The National Guard have been arriving in Washington to help protect the event amid concerns there could more violence after the US Capitol riots of two weeks ago.
Security in Washington has been ramped up after the US Capitol riots
The number flooding into the city is at least two and a half times that of previous inaugurations.
It comes after the FBI said last week that it had separately identified more than 200 suspects threatening violence at the ceremony and had picked up an “extensive amount of concerning online chatter”.
Members of the military are routinely checked for extremist links, but the FBI screening is an extra layer and shows the meticulous planning to secure the event.
Most of the National Guard on duty for the big day will be armed.
Officials told the AP news agency that the task had started more than a week ago and is scheduled to be done by Wednesday, when Joe Biden is sworn in.
“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,” said Mr McCarthy.
He added that members of the military were among the crowd on the day of the Capitol riot but it’s unclear how many were involved in the storming of the building.
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New footage published by the New Yorker gives a close-up view of the chaos of the US Capitol riot
So far only a few current service members or National Guard personnel have been arrested in connection with the violence.
On Sunday, police, military and National Guard staff were involved in a security rehearsal, with commanders lining tables around a large colour-coded map of Washington.
The potential for a bomb or coordinated armed attack is the main concern – with many Trump supporters, some with far-right links – still believing his baseless campaign that Mr Biden’s election was down to fraud.
So far, the run-up to the inauguration has been peaceful with no reports of violence despite previous FBI warnings there could be disorder at all 50 statehouses.
Small groups of protesters – some armed – gathered outside statehouses on Sunday but were outnumbered by police and National Guard.
Armed protesters outside the Oregon state capitol – but protests in the US this weekend were small and trouble-free
Many state capitols had extra security in case of trouble, such as here in Michigan
Some of the buildings had boarded up windows and installed new fences in preparation.
In New Mexico however, an official was arrested after allegedly saying he planned to take firearms to Washington to protest.
Cuoy Griffin, founder of a group called Cowboys for Trump and a county commissioner, was arrested on Sunday on charges relating to the Capitol riot, according to Justice Department documents.
He is said to have returned to New Mexico after the violence and told a council meeting that he intended to go back to DC with a revolver and rifle.
He was arrested in Washington and charged with knowingly entering or remaining in any restricted building or grounds without lawful authority.
It’s not known whether he was armed when he was detained.