WASHINGTON >> Emboldened by President Donald Trump’s campaign platform of law and order, militia groups have bolstered their strength before Election Day by attracting military veterans who bring weapons and tactical skills viewed as important to the organizations.

The role of veterans in the newly proliferating militia groups — which sometimes are steeped in racism and other times steeped simply in anti-government zealotry — has increased over the last decade, said a dozen experts on law enforcement, domestic terrorism and extremist groups.

Although only a small fraction of the nation’s 20 million veterans joins militia groups, experts in domestic terrorism and law enforcement analysts estimate that veterans and active-duty members of the military may now make up at least 25% of militia rosters. These experts estimate that there are some 15,000 to 20,000 active militia members in around 300 groups.

But gauging the size of these groups is difficult and imprecise, because much of their membership is limited to online participation. The estimates are based on samplings of militia member data gleaned from social media profiles, blogs, online forums, militia publications, interviews, assessments from watchdog groups and news reports.

At least four recently formed violent organizations were founded by military veterans, and many high-profile episodes stemming from militia groups — the killing of a federal security officer in May in Oakland, California; a thwarted plan to incite violence at a recent demonstration in Las Vegas; and the violence during a 2017 protest in Charlottesville, Virginia — involved veterans.

Underscoring how the threat of violent domestic groups is rising with limited official oversight, the top leaders of the Department of Homeland Security directed agency analysts to play down threats from white supremacist groups, according to a whistleblower complaint released Wednesday.

While militias and other paramilitary groups have been historically hostile toward the federal government regardless of the party in power, many have turned their animus in recent months toward Black Lives Matter activists as well as local leaders who enforced restrictions to combat the coronavirus. A notable example was in Michigan, where protesters, some armed, stormed the Statehouse this spring in opposition to pandemic rules. Some have begun adopting the language Trump uses to preemptively cast doubt on the outcome of an election.

Militias have historically risen after periods of war, said Kathleen Belew, an assistant professor of history at the University of Chicago and author of “Bring the War Home: The White Power Movement and Paramilitary America.”

“We have seen veterans and active-duty members being recruited because they have operational skills that are useful,” Belew said. She described the estimates of how many veterans had been drawn to the movement as “deeply concerning.”

It is an issue that federal agencies have largely avoided. “The VA has no authority to enact or enforce policies regarding veterans’ memberships in any organizations,” said Christina Noel, a spokeswoman for the Department of Veterans Affairs.

One of the larger groups, the Oath Keepers, makes recruiting veterans and law enforcement officers central to its mission.

“As a country we have spent so long at war overseas that a small percentage of veterans, but a percentage nonetheless, has warmed them to the idea that the way to deal with political conflict is to engage in armed struggle,” said Devin Burghart, executive director of the Institute for Research and Education on Human Rights, a Seattle-based research center on far-right groups. “This is a dangerous indicator of where things may go.”

From the years after the Vietnam War to the mid-1990s, a small flurry of militia groups cropped up around the United States.

Frazier Glenn Miller, a former Army master sergeant who served two tours in Vietnam, created the White Patriot Party in the 1980s. Decades later, he was sentenced to death for killing three people outside a Jewish community center in Overland Park, Kansas. In 1995, Timothy J. McVeigh, a former Army soldier who belonged to a right-wing survivalist group based in Michigan, blew up a federal building in Oklahoma City, killing 168 people, including 19 children. McVeigh promoted the works of William Pierce, who ran a white supremacist group that once posted a recruiting notice on a billboard outside Fort Bragg, North Carolina.

But beginning in 2009, antagonism toward the presidency of Barack Obama, combined with a new crop of post-Sept. 11 veterans, fueled exponential growth in militia groups.

While the military strictly forbids its active-duty personnel from participating in hate groups, it is silent on militias and the role of veterans who have left service.

“Veterans are often looked at for their paramilitary skills, their ability to survive in the field as well as leadership skills,” said Daryl Johnson, a former senior terrorism analyst at the Department of Homeland Security. “They are proficient with weapons, which they often own.”

While many veterans who are deployed overseas return filled with gratitude to be back in the United States, others return with very different views, informed by their work in countries whose political systems they despise and fearful that such ideologies could infiltrate their own country.

“You see overseas how things can go wrong,” Johnson said. Fear of communism, Islamic law and Marxism permeate some veterans’ thinking. “They take experiences they have had overseas and transport them to the homeland and think there are all these threats,” he said.

In 2009, the Department of Homeland Security released an intelligence assessment warning that returning veterans who faced trouble reintegrating could “lead to the potential emergence of terrorist groups or lone wolf extremists capable of carrying out violent attacks.”

The report led to such an outcry from conservatives and one prominent veterans organization that the department deep-sixed it. “We used the term ‘disgruntled,’ so that terminology was insensitive,” said Johnson, who helped prepare the report. “We were trying to show they were susceptible to recruitment because of skills they learned. That is a glaring truth no matter who is offended.”

That same year, the FBI did its own investigation of extremist groups with a focus on veterans from Iraq and Afghanistan.

The Obama years were a growth period for these groups, many of them loosely tied to the Tea Party movement. Most notable was the Oath Keepers, formed in 2009 with a core notion that its members should continue to honor the oaths they took in the military and law enforcement agencies to defend the country, via their efforts in a militia.

Stewart Rhodes, a former Army paratrooper who served as a staff member for Ron Paul, then a Republican representative of Texas, “formed the group to encourage current and former military and law enforcement members to honor their oath against tyranny,” said Sam Jackson, an assistant professor at the University at Albany in New York who has written a book on the group. “But the focus of threats has changed to be antifa and Black Lives Matter and others on the left.”

The movement has accelerated during Trump’s time in office. In 2015, Brandon Russell, a member of the Florida Army National Guard, formed the Atomwaffen Division, a neo-Nazi group. One of its members, Vasillios Pistolis, a private at the time, participated in the “Unite the Right” rally in Charlottesville, bragging on social media about injuring people. (He was later kicked out of the Marines.)

After that rally in 2017, Joffre Cross III, a former private in the 82nd Airborne Division at Fort Bragg and a member of the newly formed Patriot Front, was charged with multiple weapons felonies.

The “boogaloo” movement, a loose network of right-leaning, anti-government groups that seeks to bring about a second civil war to overthrow the government, has been around since 2012, when it was largely an online movement.

In June, Daniel Austin Dunn, a former Marine, was indicted in Texas for making violent threats toward police officers on Facebook and Twitter posts, in which he associated himself with boogaloos. Authorities found a cache of weapons at his house. This year, the FBI arrested an Army reservist and two veterans with ties to the movement for planning to incite violence at a Las Vegas protest. An active-duty airman affiliated with the group was also charged with killing a federal officer in Oakland.

A small number of veterans have joined ranks with left-leaning groups or groups not associated with the political right. A sniper who shot a dozen Dallas police officers in 2016, killing five, was an Army veteran.

The man law enforcement officials believe shot and killed a right-wing activist in Portland, Oregon, last month was an antifa supporter and a veteran; he was killed last week by police. But veterans with far-left views are small in number and tend to act outside any organized force — the antifa movement, for example, lacks the structure and leadership of a militia — according to experts in the field.

Many groups have proclaimed themselves as enforcers of Trump administration policies and, more recently, as protectors of businesses in cities with protests, often antagonizing those protesters. The confrontations with protesters have also dovetailed with actions to protest coronavirus containment measures, often with a side of conspiracy theories to generate new member interest.

A well-known group, the Three Percenters, focuses on anti-immigrant activities and targets leftists like members of antifa. A leader of a chapter in Georgia, Chris Hill, is a Marine veteran who leads regular field training exercises.

The United Constitutional Patriots, a militia that patrols the southwestern border with Mexico, has also attracted veterans.

“The militia movement traditionally hated the federal government,” said Heidi Beirich, a co-founder of the Global Project Against Hate and Extremism. “This has completely changed with Trump.”

As they have inserted themselves in cities with large protests, the groups have found themselves sometimes welcomed by local law enforcement. “We have militia groups that are inserting themselves into cities to, from their perspective, to fill a vacuum of law enforcement,” said Seth G. Jones, a senior adviser at the Center for Strategic and International Studies. “But they are doing things outside of the law to take law and order into their own hands.”

Mike Martinez, police chief of Arroyo Grande, California, said the militias were a concern. “Many militias have their own ideology,” he said. “Some are not pro-law enforcement, so it is always important for us to be aware.”

The end of the Trump era would not spell the end to militias, the experts agreed. “In the immediate aftermath of an election, I don’t see this ebbing,” Jones said. “In fact my concern is there will be a range of organizations that don’t support the legitimacy of a Biden president, and that administration will have to think about how to disarm militias. That will be a dangerous situation.”