Angry, desperate, and firm in their belief that they're above the law, America's "sovereign citizens" are presenting a mounting threat to domestic law and order, according to reports and terrorism experts.

While the sovereign-citizen movement has been around for decades – evolving from a 1970s antitax protest to the "freeman" movement of the 1990s – it has spread in recent years, driven by high unemployment, mass foreclosures, and other economic hardships. The lure is the movement's peculiar ideology: that Americans can free themselves from the authority of the United States government through obscure and byzantine legal filings that purport to reveal the illegitimacy of US law.

Jared Loughner, the suspect in the Feb. 8 shooting of Rep. Gabrielle Giffords and 18 others in Tucson, Ariz., used "sovereign citizens" talking points, and two well-known sovereign citizens shot to death two West Memphis, Ark., police officers in May 2010.

On Wednesday, Mr. Loughner pleaded not guilty to a 49-count indictment in a Tuscon courtroom. Fourteen of the charges carry the possibility of the death penalty, though prosecutors have not yet decided whether to seek Loughner's execution.

The incidents in Arizona and Arkansas have followed others:

Joe Stack, who piloted a Piper Dakota airplane into an Austin, Texas, IRS building in February 2010, was a sovereign citizen who left a convoluted manifesto.

The Guardians of the Free Republic, a sovereign-citizen group that has tried to set up an alternative US government, last year sent subtly threatening letters to all 50 US governors.

Scott Roeder, convicted of fatally shooting late-term abortion doctor George Tiller at a Sunday church service in Wichita, Kan., in May 2009, has links to the sovereign-citizen movement.

A report released last week by the Southern Poverty Law Center cites a "dramatic increase" in sovereign activity, estimating about 300,000 adherents nationwide. The trend marks a reprise of the early 1990s, when fears and anxieties about the economic downturn swelled the movement to about 250,000 followers and culminated in the FBI's 81-day armed standoff with a group of Montana freemen in 1996.

Once populated mostly by white men, the sovereign-citizens movement is now drawing in women, Hispanics, and African-Americans, according to the Southern Poverty Law Center – including a surprising number of doctors and policemen. They are involved in a wide variety of schemes and scams, experts say, and the refusal by some to show deference to US law has led to dramatic acts of violence.

"There's been a shift in recent months on law-enforcement Internet forums, where they used to joke about them and now it's, 'I had to encounter one of these people today, and I was careful,' " says J.J. MacNab, a tax expert and author in Maryland who has been tracking the sovereign-citizen movement for a decade. "And they should be careful, because these people are very heavily armed, they're angry, and they believe they're right."

What do sovereign citizens believe?

The basic sovereign-citizen notion is that the United States has secretly enslaved its people – and that sovereign citizens alone know it. The theory goes like this: At some point in the past, the US replaced the original "common law" of the Founding Fathers with a system of law that brings Americans into the servitude of the federal government. Moreover, when it ended the gold standard in 1933, the US government used its citizens as collateral. In place of gold, the US leveraged its people's future earnings to lure foreign investors, meaning that Americans were essentially being sold at birth.

The alleged conspiracy goes deeper. As a part of this enslavement, the government establishes a secret Treasury account corresponding to every American citizen, which it fills with as much as $20 million. This fake doppelganger – which the movement refers to as a "straw man" – is how the government enslaves the flesh-and-blood American. Names on government-issued IDs are printed only in capital letters, sovereigns say, because they actually refer to the straw men, not the real people (whose names, after all, are not spelled only in capital letters).

The sovereign's goal is "redemption" – becoming a "free man" liberated from the straw man and slavery to the US government. Most often, sovereign citizens attempt this through "paper terrorism." Some file massive amounts of legal documents with dense, nonsensical language. Some issue fake million-dollar bonds, sell phony diplomatic credentials, impersonate police officers, or use fake driver's licenses and birth certificates.

Some have also threatened judges. In tough economic times, many have turned to debt- and mortgage-elimination scams that appeal to people who fall for get-rich-quick schemes.

In Atlanta, the city has seen at least 30 cases of sovereign citizens stealing empty foreclosed homes by filing illegal quit-claim deeds on those properties. In one case, a man seized an entire foreclosed strip mall.

Detectives and dentists

Similar anecdotes are common nationwide and point to the spread of the movement.

In Florida, a local doctor is hoping to set up the Republic of Florida, a new country untouchable by US law. Last month, the Sarasota Police Department fired a prominent detective after he filed court documents renouncing his US citizenship, signing each court document with a red thumb print – the imprimatur of sovereign citizens. And Tucson shooting suspect Jared Loughner in his YouTube videos used phrases about literacy and the gold standard that occur frequently in freemen jargon.

"You used to see a sovereign story once a month; now we're seeing three or four a day," says Ms. MacNab. "Most people when they think sovereigns think rural, uneducated, ex-military, but dentists fall for this stuff in droves."

At times of economic and societal unrest, such conspiracy fantasies are fueled by mainstream concerns. "These are always contorted, mirror reflections of anxieties that exist in the mainstream, and that's where they get [the] loose cherries that fall off the tree," says Brian Levin, director of the Center for the Study of Hate and Extremism at California State University, San Bernardino. "And when conspiracy theories are more prevalent, that tree shakes even more."

He adds: "Unlike people who refute the moon landing, these are people doing it for something beyond ideological vanity points. They believe that the political and economic institutions and processes are illegitimate, and in some ways they can actually point to real inequities" in the system.

A 'modern-day freedom fighter'

In an interview with ABC News last year, self-described sovereign citizen Brian Johnson asserted that he believes that Americans don't need Social Security numbers, driver's licenses, or even wedding licenses.

"I call myself a modern-day freedom fighter," said Mr. Johnson. "You're the ruler, the master in your life. I'm not a danger to anyone, except those who don't wish to have the truth exposed."

Oklahoma City bombing accomplice Terry Nichols was also a sovereign citizen.

The US government has taken steps to thwart tens of thousands of sovereign-citizen-related filings every year, including a $5,000 fine for filings that include the straw-man argument. Last year, the IRS warned that "attacks and threats against IRS agents" have risen in recent years.

In a recent NPR report about secret prisons in the United States, at least two inmates at a federal Communications Management Unit in Indiana – which limits the amount of communication prisoners have with the outside world – were prominent sovereign citizens.