In 2014, militias from around the country rallied to support the Nevada ranching family in the biggest armed standoff against the government in decades. In the desert town of Bunkerville, they saw themselves as part of a brand-new movement, united in defiance of the federal government.

But the patriots, as they call themselves, have roots in an anti-government militia movement that emerged in the 1990s from bloody confrontations with federal agents. Today, the movement has evolved: they’re savvier, better organized — and they’ve stepped into the mainstream, with websites, Facebook pages and public events.

The new militias have filled their ranks by drawing on populist anger toward the government, which escalated with the inauguration of Barack Obama, as well as on anti-immigrant or anti-Muslim sentiment. The number of anti-government militias more than doubled between 2009 and 2010, to 330.

“There’s been a smarter approach in terms of taking the ideology and moving it out of the shadows,” said Ryan Lenz a reporter for the Southern Poverty Law Center, which tracks extremist groups. “And I think it is consistent with a migration we’ve seen in a lot of extremist ideologies, including racist ones … And the political environment, such as it is, has proven to be marvelously, marvelously accepting of these ideas.”

Law enforcement sees them as a serious threat. Nearly 74 percent of state and local law enforcement agencies in a 2015 survey said that anti-government violent extremism is the biggest threat they face in their communities — far higher than the threat from Muslim extremists, white supremacists, eco-terrorists or other politically motivated violence. These incidents don’t always make headlines. But between 1990-2014, there were actually more deadly attacks by far-right extremists than by Islamic extremists in the U.S., according to data from a government-funded database that tracks incidents.

Part of the reason is that these individuals are well-armed and well-trained, and it’s not always easy to predict when — or even if — they’ll turn to violence. “Not every single [militia] group is hell-bent on overthrowing the government or wanting to instigate terrorism,” said Daryl Johnson, the former head of the Homeland Security department’s domestic extremism program. “[But] you’re always running that risk of someone coming into contact with your group being exposed to these, I would say, radical extreme far-right beliefs … then going out and doing something criminal.”

For the FBI, which takes the lead on domestic terrorism investigations, that’s one of the biggest challenges, and dangers, in dealing with the movement. Here’s a quick guide to how they got started — and where they may be headed.