More than 21,000 National Guard troops assumed positions across the heart of the nation’s capital on Monday as military and law enforcement officials took new steps to head off any insider threats among the ­massive force of part-time soldiers during President-elect Joe Biden’s swearing-in this week.

As part of an intense inauguration security effort following the deadly Jan. 6 riot at the U.S. Capitol, authorities have launched a process to subject troops from the D.C. National Guard and a host of states to additional security screening for links to extremist or other violent groups.

Concern about potential internal threats has intensified as investigators have identified a growing list of people with law enforcement and military ties, including at least two service members, among the rioters who stormed Congress in their effort to overturn President Trump’s electoral loss.

On Monday, an Army reservist from New Jersey became the latest service member known to be charged with participation in the insurrection, which left a police officer and four rioters dead and disrupted lawmakers’ attempt to certify Biden as the next U.S. president.

Now, the Defense Department, assisted by the FBI, will conduct special vetting for National Guard personnel in Washington, who are expected to number up to 25,000 for the inauguration on Wednesday, said acting defense secretary Christopher C. Miller.

“While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital,” Miller said in a statement. While law enforcement vetting typically occurs before major security events, he added, “in this case the scope of military participation is unique.”

An FBI official said the bureau was conducting “name checks” for the guardsmen in addition to the vetting done by the Defense Department. It was not immediately clear whether the military or state National Guards are doing any new or additional screening of the guardsmen, who have been previously vetted as part of their recruitment and security clearance process.

The presence of camouflaged guardsmen flanking Washington’s federal heart, largely closed off with steel fences and concrete crash barriers, has added to the sense of tension across the city in the days since the Capitol riot.

Even as expected follow-on protests have failed to materialize, authorities have promised to keep a high alert throughout a scaled-back inauguration.

The heightened scrutiny of guardsmen highlights the Pentagon’s concerns about growing extremism, including support for white nationalism and anti-government groups, among veterans and serving troops. Authorities acknowledge that rooting out the problem has proved challenging, which they say reflects larger trends across American society.

Before the riot, as part of the military’s reckoning with its legacy of racism and discrimination Miller ordered a review of how the military handles extremism in the ranks. Officials are hoping to determine whether existing steps to identify and address extremist affiliations and actions are adequate.

On Monday, the Army confirmed that Timothy Hale-Cusanelli, whom the Justice Department has charged with five counts including obstructing a law enforcement officer during a civil disorder, serves as a sergeant in the Army Reserve. Task and Purpose first reported the man’s Army affiliation.

According to a charging papers, a source identified Hale-Cusanelli, who works as a contractor for the Navy at Naval Weapons Station Earle in New Jersey, as an “avowed white supremacist and Nazi sympathizer” and said he admitted entering the Capitol as part of the insurrection. Hale-Cusanelli told the source he directed others to advance and admitted to stealing a flagpole that was used by another rioter to attack a member of the Capitol Police, the paper said.

According to the Army, Hale-Cusanelli, a human resources specialist, serves in the 174th Infantry Brigade out of Joint Base McGuire-Dix-Lakehurst in New Jersey.

“The Army does not tolerate racism, extremism or hatred in our ranks and is committed to working closely with the FBI as they identify people who participated in the violent attack on the Capitol to determine if the individuals have any connection to the Army,” a spokesman for Army Reserve Strategic Communications said.

Spencer Hsu, Matt Zapotosky, Carol D. Leonnig and Devlin Barrett contributed to this report.