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Tapping on his cellphone with a sense of purpose, Kevin Mathewson, a onetime city alderman in Kenosha, Wis., dashed off an online appeal to his neighbors. It was time, he wrote on Facebook to “take up arms to defend our City tonight from the evil thugs.”

One day earlier, hundreds of residents had poured onto the streets of Kenosha to protest the police shooting of 29-year-old Jacob Blake. Disturbed by the sight of buildings in flames when he drove downtown, Mathewson decided people needed to arm themselves to protect their houses and businesses.

To his surprise, some 4,000 people responded on Facebook. Within minutes, the Kenosha Guard had sprung to life.

His call to arms — along with similar calls from others inside and outside the state — propelled civilians bearing military-style rifles onto the streets, where late that night a gunman scuffling with protesters shot three of them, two fatally. The Kenosha Guard then evaporated just as quickly as it arose.

Long a divisive figure in Kenosha, Mathewson, 36, does not fit the typical profile of a rifle-toting watchdog, although he said he supported President Donald Trump on Second Amendment grounds. The rise and fall of his Kenosha Guard reflects the current spirit of vigilantism surfacing across the country.

Organizations that openly display weapons have existed for decades. Anti-immigrant and Islamophobic sentiments are rife, and some militant groups, like the Oath Keepers or the Three Percenters, train together under established hierarchies. Ever since the 2017 white nationalist march in Charlottesville, Va., armed groups have become fixtures at demonstrations around the country, although membership numbers remain opaque.

With the approaching election ratcheting up tensions, armed groups that assembled via a few clicks on the keyboard have become both more visible and more widespread. Some especially violent groups are rooted in long-standing anti-government extremism, like the 14 men charged with various crimes in Michigan this month. They included six accused by the FBI of plotting to kidnap Gov. Gretchen Whitmer, whom the suspects had labeled a “tyrant.”

Starting in April, demonstrations against lockdowns prompted makeshift vigilante groups to move offline and into the real world. That trickle become a torrent amid the nationwide protests after the police killing of George Floyd in Minneapolis — with some armed groups claiming to protect the protesters while others sought to check them.

“They just spawn out of nowhere,” said J.J. MacNab, a fellow at George Washington University’s Program on Extremism.

When Trump was asked at last month’s debate about activity by right-wing extremists, he declined to outright condemn such groups. His response — telling one far-right group to “stand back and stand by” — left experts who had warned about the potential for greater violence before the election bracing for more.

Experts who study violent groups say that many are technically not militias; they are too unstructured. They are usually just a fraternity with a shared goal, like the groups in Oregon that patrolled back roads amid wildfires, hunting mostly imagined looters or arsonists.

In Kenosha, police officers were caught on video expressing appreciation to the gunmen and handing them bottles of water, prompting criticism that law enforcement officers encouraged the armed groups.

But soon after, the sheriff tried to distance his department. “Part of the problem with this group is, they create confrontation,” Kenosha County Sheriff David Beth told reporters. Asked later about any investigation, the Sheriff’s Department said it had not referred any cases linked to the Kenosha Guard for prosecution; the Police Department didn’t respond.

The Second Amendment may mention a “well-regulated militia,” but all 50 states ban private paramilitary groups, said Mary McCord, a former senior Justice Department official now at Georgetown University Law Center. Some groups avoid calling themselves “militias” to circumvent the prohibition, experts said.

Mathewson first tried to muster the Kenosha Guard in June after the city had small protests following Floyd’s death in Minnesota. A little more than 60 people responded.

Then, on Aug. 23, video emerged that showed a Kenosha police officer firing seven times toward Blake’s back.

When protests disintegrated into property destruction, Mathewson said, he thought law enforcement was overwhelmed. “Some people think that we should have left it up to the police, but I disagree. They were outnumbered. Our leaders failed us,” he said.

After two nights of demonstrations, he posted an event on Facebook called “Armed Civilians to Protect our Lives and Property.” He named himself commander of the Kenosha Guard and added an open letter to police telling them not to interfere.

“That got a lot of traction,” Mathewson said. “Tons and tons of shares, likes, comments. I was receiving private messages to my Facebook page, my public figure page. They were just raining in.”

Several hundred people volunteered, and around 4,000 expressed approval. His call to arms spread to other platforms, like Reddit. Infowars, the website that traffics in conspiracy theories, amplified it, as did right-wing radio.

Mathewson said he had long believed that Americans should protect themselves. “You cannot rely on the government or the police to protect you,” he said.

Before forming the Kenosha Guard, he had seen reports focused on armed groups deploying in Minneapolis and Portland, Ore. “It was so far from me that it did not seem real,” he said. “When it happens in your own backyard, your own city, it is like, ‘Jeez, what can I do?’

“I am pro-Second Amendment, but I am not a right-wing nut job,” he added.

Posts on Facebook amplified the sense of siege in Kenosha by spreading false rumors that murderous gangs from Milwaukee, Minneapolis and Chicago were coming to ransack the city of 100,000 people.

Jennifer Rusch, 47, a hairstylist, clicked on Mathewson’s web page to find armed men to protect her business. “Facebook had a lot to do with making everybody hysterical,” she said. “Now we know 99 percent of it was lies.”

People messaged Mathewson from around Wisconsin and other states, asking where to deploy. He could not handle the responses flooding his cellphone, he said.

“People thought we had some kind of command staff or a structure, but it was really just a general call to arms” meant mostly for his neighbors, Mathewson said.

Jerry Grimson, 56, a former campaign manager for Mathewson during his run for alderman, responded by organizing his own neighbors to come out. “There was no way we were going to let people burn down our homes,” he said.

That night, Mathewson stuck to the entrance of his subdivision, WhiteCaps, at least 7 miles from the city center. Pictures show him wearing a baggy red Chuck Norris T-shirt and knee-length camouflage shorts, with a rifle slung over his chest. He passed the early evening sitting outside on a lawn chair with some armed neighbors, then went to bed early. “I kind of felt a little bad that I got this in motion but then I was home by 9,” he said.

While he slept, downtown Kenosha boiled over.

Witnesses blamed the violent disarray partly on the fact that many gunmen downtown were strangers to one another, with some on rooftops acting as spotters to call in re­inforcements and no one in command.

To Raymond Roberts, a real estate investor and six-year Army veteran who monitored the vigilantes, the parade of jacked-up pickup trucks filled with armed men resembled Afghanistan. “You have guys in the back of trucks, faces hidden, and you can tell that they are hunting,” he said.

Roberts noticed that law enforcement officers largely ignored the men.

The gunmen never seemed to realize that all the combat weaponry made Black residents like himself particularly uneasy, Roberts said, and that the community would have preferred to protect itself. “They just had this assumption that we don’t exist,” he said.

As tensions surged, with protesters and armed enforcers tussling, authorities say that Kyle Rittenhouse, 17, from nearby Illinois, opened fire with a military-style semiautomatic rifle, killing two protesters and seriously wounding a third. He faces homicide charges and has become a poster boy for the far right.

Mathewson remains unsure which armed men downtown responded to his call, and he denied having any contact with Rittenhouse.

Angie Aker, a community activist, initiated a criminal complaint against Mathewson as an accessory to the protest deaths. There is also a federal lawsuit that names Mathewson, along with Rittenhouse and Facebook, among others, for depriving the four plaintiffs of their civil rights; one is the partner of a victim, and the three others allege that armed men assaulted them.

After the shootings, Facebook banned Mathewson for life, removing his personal and professional pages. He said he lost 13 years of photo archives and a memorial page for his mother. Mathewson said that for now he has no plans to revive the Kenosha Guard.