The FBI will vet all 25,000 National Guard troops coming into Washington DC for President-elect Joe Biden’s inauguration, it has been announced.
The decision was made as US defence officials became worried about the possibility of an insider attack or other threat from service members involved in securing the event.
At least two-and-a-half times the usual number of National Guard troops have been deployed compared to previous inaugurals.
The vetting is in addition to routine reviews of service members for any extremist connections.
The process began as the first troops began deploying to DC more than a week ago, following the storming of the US Capitol building by pro-Trump protesters.
The United States Secret Service is in charge of event security, but there is a wide variety of military and law enforcement personnel also involved, ranging from the National Guard and the FBI to Washington’s Metropolitan Police Department, the US Capitol Police and US Park Police.
Acting US Secretary of Defense Christopher C Miller said: "As is normal for military support to large security events, the Department will vet National Guardsmen who are in Washington DC.
"While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital. This type of vetting often takes place by law enforcement for significant security events.
"However, in this case, the scope of military participation is unique," he added.
Army Secretary Ryan McCarthy said officials are conscious of the potential threat and 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.
He added National Guard members are also getting training on how to identify potential insider threats.
The riot at the Capitol on 6 January came hours after a 'Save America Rally' in which President Donald Trump spoke for more than an hour, airing grievances against the election, the media and the Democrats.
According to Mr McCarthy, service members from across the military were at the 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 riot which left five people dead.
General Daniel R. Hokanson, chief of the National Guard Bureau, said: "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 by the chain of command immediately."
Mr McCarthy added the key goal is for the transfer of power to happen without incident.
"This is a national priority," he said. "We want to send the message to everyone in the United States and for the rest of the world that we can do this safely and peacefully."
Cover image: National Guard helmet in the grounds of the US Capitol (Picture: USA Today Network/SIPA USA/PA).