TAMPA — Brandon Russell sat quietly in a federal courtroom Thursday, dressed not in the uniform he often wears as a private first class in the Florida National Guard but in an orange Pinellas County jail jumpsuit.

A self-professed neo-Nazi leader, the 21-year-old Tampa Palms man faces explosives charges after authorities searching his apartment found bomb-making materials. He spoke in chat rooms about killing people and attacking infrastructure, investigators said.

Any connections between Russell's extremist leanings and his work in communications with the National Guard are still unfolding. But there's no question about the appeal military experience holds for those whose cause is racial violence.

"White supremacist and neo-Nazi groups encourage their members to sign up to the U.S. military," Matthew Kennard, an author who has written on extremism and the military, told the Tampa Bay Times. Their goal: "To get free training for the racial holy war — 'rahowa' — they want to fight back in the U.S."

Heidi Beirich, who leads the Intelligence Project of the Southern Poverty Law Center, said the Russell case "is an example that the military needs to be very, very serious about this issue."

No one can say for certain how many among the nation's 2.2 million uniformed personnel are, like Russell, actively involved with extremist groups. But all agree the numbers are small.

Kennard, who interviewed neo-Nazi leaders for his book Irregular Army, said their estimate is in the low hundreds — similar to the numbers from an FBI analysis of white supremacist incidents from 2001 to 2008. Other serious analysts, Kennard said, put the number as high as 1,000.

One study found that nearly 7 percent of those arrested in connection with jihadi incidents had U.S. military experience.

And a recent study in Justice Quarterly suggests that nearly 20 percent of extremists in neo-Nazi and other white power groups, antiwhite groups and even jihadi organizations, have served in the military.

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Authorities first encountered Russell while investigating the shooting deaths May 19 of two of his roommates. A third roommate, 18-year-old Devon Arthurs, is accused in the slayings, saying he fired the fatal shots to keep the other young men from committing acts of domestic terrorism.

In searching Russell's belongings, investigators said, they found some of the icons of white supremacists — copies of Adolf Hitler's Mein Kampf, the apocalyptic novel The Turner Diaries, and a framed photo of Timothy McVeigh, the former National Guard soldier and white supremacist executed for detonating the truck bomb that killed 168 in Oklahoma City in 1995.

Extremism in the military has ebbed and flowed over the years, rising in the 1990s and again with the election of Barack Obama, the nation's first black president, according to a 2009 study on right-wing extremism by the U.S. Department of Homeland Security.

RELATED: Court hearing for Devon Arthurs, accused jihadist charged in Tampa Palms double murder

The conditions spelled out in the report seem familiar eight years later.

Right-wing extremist chatter on the Internet focused "on the economy, the perceived loss of U.S. jobs in the manufacturing and construction sectors, and home foreclosures," the report says. "Anti-Semitic extremists attribute these losses to a deliberate conspiracy conducted by a cabal of Jewish 'financial elites.' "

The report says that over the years, immigration concerns were frequently cited as reasons for extremist beliefs:

"Right-wing extremists were concerned during the 1990s with the perception that illegal immigrants were taking away American jobs through their willingness to work at significantly lower wages."

White supremacists represent just one of the extremist groups with connections to the military.

Last summer, Army Reserve Pfc. Micah Xavier Johnson, who was associated with black power groups, opened fire on police officers in Dallas, killing five and wounding nine others.

Johnson wanted to target white people, especially police, Dallas Chief David Brown told reporters at the time. Before his rampage, Johnson posted a rant against white people on a black nationalist Facebook group called Black Panther Party Mississippi, denouncing the lynching and brutalizing of black people, according to Newsweek.

Some jihadis also have undergone U.S. military training.

A recent study by the New America Foundation think tank found that in 28 out of 406 jihadist cases examined, individuals involved had U.S. military experience, David Sterman with New America told the Tampa Bay Times via email.

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Brandon Russell seems to fit a pattern the Army turned up while investigating soldiers it considers domestic terrorists and extremists, said one former Army criminal investigator.

From 2012 to 2015, there were 22 domestic terrorist and extremist investigations by the Army Criminal Investigations Division, said Carter F. Smith, a former investigator and manager with the Army division and now a professor at Middle Tennessee State University.

Fifteen subjects were identified, including 12 soldiers — all on active duty, mostly white males 20 to 24, and single enlisted members, Smith said.

The military maintains that the number of extremists in its ranks is minuscule but agrees that even a few can be a problem.

"The overwhelming majority of service members are honorable, law-abiding, disciplined, patriots who represent the very best of America's population," said Army Lt. Col. Myles Caggins, a Pentagon spokesman.

"The Defense Department recognizes the potential harm from outliers who affiliate with fringe groups that promote ideologies that are inconsistent with our values."

In November 2009, the Pentagon tightened its policy on extremists, according to the Southern Poverty Law Center, a civil rights legal advocacy group. New rules specified that troops cannot actively participate in hate groups, including rallying, training, organizing or distributing supremacist material.

The new rules "should make it easier for commanders to investigate and discharge" extremists, according to the center.

Of particular concern, said Smith, the former Army investigator, are findings like those from Justice Quarterly about how many extremists have military experience.

Smith says he thinks most estimates under count how many extremists serve in the military but he sees the greater danger as what happens when they take off the uniform.

"Sadly, this is not a huge issue on the radar of the military, and only gets there when it causes really bad press."

He added, "It's hard to persuade folks in the military that they need to be as concerned about their service members when they get out as they were when they came in."

Contact Howard Altman at haltman@tampabay.com or (813) 225-3112. Follow @haltman.