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 anti-tax 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 U.S. 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 U.S. government, last year sent subtly threatening letters to all 50 U.S. 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 U.S. 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 U.S. 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 U.S. government used its citizens as collateral. In place of gold, the U.S. 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).