Vem Miller, the individual taken into custody at a checkpoint in Southern California while reportedly heading to a Donald Trump campaign event, has been linked to the extremist movement known as the Sovereign Citizens, according to the local sheriff.

Miller’s arrest came after officers challenged him at a checkpoint and observed that his vehicle had an unusual license plate. Riverside County Sheriff Chad Bianco informed the media that this homemade plate is a sign of affiliation with those who consider themselves sovereign citizens.

The Secret Service and other federal agencies confirmed that Trump was not in any danger during this incident. Miller has since been released on bail set at $5,000.

So, what exactly is the Sovereign Citizen movement? Why might law enforcement express concern regarding anyone affiliated with this group?

Defining the Sovereign Citizens

The Sovereign Citizen movement is a loosely connected assortment of anti-government believers—possibly numbering in the thousands or even tens of thousands—who contend that they are not bound by federal or state laws. Many within this group, for example, refuse to pay taxes, and there have been numerous notable cases where they have attempted to present their beliefs in court, usually without success.

Unlike structured organizations such as the Oath Keepers or the Proud Boys, the Sovereign Citizen movement functions more as a collective ideology, according to Mark Pitcavage, a senior researcher at the Anti-Defamation League’s Center on Extremism.

“They are staunchly anti-authoritarian,” said Pitcavage. “They reject any form of authority that attempts to impose rules on them.”

The U.S. Attorney’s Office announced that “the U.S. Secret Service finds that this incident did not affect security measures and the former president was not at risk.”

Members of the Sovereign Citizen movement have previously engaged in acts of violence against government entities. Notably, in 2010, a pair of self-identified Sovereign Citizens attacked and killed two police officers during a routine traffic stop in Arkansas before being killed themselves.

Growth of the Sovereign Citizen movement during Trump’s presidency

While the movement does not align with any political party and traditionally eschews politics, it has nonetheless gained traction and attracted new followers during the Trump administration.

A recent report from the ADL’s Center on Extremism indicated that the Sovereign Citizen movement has been expanding in recent years, drawing in new supporters from groups like anti-vaxxers, QAnon believers, and those who align with Trump’s “Make America Great Again” message.

“The movement has also made significant inroads in jails and prisons across the United States. Sovereign Citizens have developed an international presence as well, spreading to multiple countries around the globe,” the ADL’s briefing explained.

The Sovereign Citizen ideology began to gain popularity in the late 1970s, largely fueled by various “gurus” and leaders who promote their Anti-Government beliefs, according to Pitcavage.

“They are present throughout the nation,” he noted. “It’s a considerable movement.”