An al Qaeda splinter group has declared an Islamic State in portions of Iraq and Syria and threatened to "raise the flag of Allah in the White House." US intelligence agencies suspect that the Islamic State is determined to launch a major attack against Western targets, and South Carolina Senator Lindsey Graham recently warned of "an American city in flames" as a consequence of such a strike.

So why do American law enforcement authorities regard a fringe ideology of homegrown radical nutjobs as the country's top terrorist threat, instead of Islamic extremists?

According to a recent study conducted by the National Consortium for the Study of Terrorism and Response to Terrorism (START), which surveyed 364 officials from 175 law enforcement agencies, America's top perceived terrorist threat is an ideological subculture of extreme opposition to the government known as the sovereign citizens movement. The START survey found that 86 percent of respondents either agreed or strongly agreed that sovereign citizens present a serious terrorist threat, compared to 67 percent that said so of Islamic extremists.

Not really amounting to a proper movement or cohesive group, sovereign citizens come from various political, economic, and social backgrounds, and essentially regard themselves as self-governing individuals to whom the law does not apply.

"Essentially, you have a belief system that circumvents society and the rule of law, but also endorses violence as a viable tactic to avenge grievances or to express yourself," Daryl Johnson, a former senior domestic terrorism analyst at the Department of Homeland Security and the owner of DT Analytics, a private law enforcement consulting firm, told VICE News. "Just like any political, mainstream, or religious ideology, you have people with a varying level of commitment to, and understanding of, the movement."

Self-declared sovereigns include elderly radical right-wingers who were drawn to the Posse Comitatus social movement of the 1970s (a sovereign antecedent that assailed the US government as an illegitimate Zionist stooge) as well as middle-aged anti-tax extremists and younger zealots that regard themselves as quasi-libertarian anarchists.

'The fact that you have people across America that believe that the federal government is an illegal entity, that a lot of state governments are illegal, and that the laws do not apply to them is very subversive to our rule of law and to our society.'

Steven Chermak, a professor of criminal justice at Michigan State University and one of the authors of the START report, told VICE News that domestic concern over Islamic extremism has diminished somewhat due to the lack of notable terror attacks within the US since 9/11. Meanwhile, he said, attacks by sovereigns, "especially on local law enforcement, and the media coverage tied to those events, have just impacted law enforcement's perception."

Sovereign activists were among the various groups drawn to Nevada cattle rancher Cliven Bundy's standoff with federal authorities this year over his refusal to pay roughly a million dollars in grazing fees to the Bureau of Land Management. "I abide by all of Nevada state laws," he remarked to a radio station. "But I don't recognize [the] United States government as even existing."

Jerad and Amanda Miller were among the armed protesters who rallied to Bundy's cause, though they were eventually forced from the ranch for views that were too radical even for Bundy. In June they randomly killed two Las Vegas police officers eating at a local pizzeria, laying a "Don't Tread on Me" flag over one of their bodies and declaring a revolution. The pair later died after a shootout with police at a nearby Walmart. Some commentators were quick to connect them to sovereign extremism.

This past Monday, a 60-year-old named Douglas Leguin set fire to a dumpster in Dallas before calling 911 and declaring himself a sovereign citizen. He was armed with an AK-47 and attempted to ambush police and firefighters responding to the fire before he was apprehended.

The FBI addressed the growing threat posed by sovereign extremists in 2012, issuing a counterterrorism brief that identified them as "comprising a domestic terrorist movement."

"Sovereign citizen extremists are mostly a threat to law enforcement officers who encounter them during traffic stops or during enforcement actions," FBI spokesperson Angela Bell told VICE News. She added that extremists might regard the police as enforcers of an illegitimate government. "Therefore, police officers have cause to be cautious when they encounter a sovereign citizen in the course of their official duties."

Sovereign extremists have developed a habit of targeting officers and federal officials.

Jerry and Joseph Kane, a father-son pair of self-proclaimed sovereigns, murdered two West Memphis police officers in 2010 following a routine traffic stop before being killed in a police shootout. Schaeffer Cox, an Alaskan sovereign militia leader, was convicted in 2012 of collecting illegal weapons and conspiring to kill law enforcement officers. That year, two suspects involved in an ambush of police officers in Louisiana were identified as sovereigns. The following year, two more self-declared sovereigns were arrested for planning to torture and kill Las Vegas police officers.

While Johnson noted the persistent threat of violence posed by sovereign extremists, he noted that many sovereign believers prefer to commit various white-collar crimes.

"They encourage white-collar crimes, what we consider 'paper terrorism,' like placing fraudulent liens against perceived enemies, putting out bounties and warrants for judges and police officers who they come in contact with, and things like that," he said. "This can be particularly intimidating to a police officer's or judge's family because these people will stalk you, find out where you live, and issue these threats."

Other sovereign extremists facing federal tax charges have been convicted of plotting to kill federal judges presiding over their cases.

Bell, the FBI spokesperson, emphasized that it's not illegal to identify oneself as a sovereign citizen or hold sovereign beliefs.

"The FBI only becomes involved when a sovereign citizen breaks the law," she said. "In the overall sovereign citizen movement, violence is fairly rare, and until a sovereign citizen commits alleged criminal violations they are not investigated by the FBI. The FBI does not investigate First Amendment-protected activities."

Tea Party groups have criticized the concern over sovereign extremism as a covert means of reigning in conservative activism.

"The Obama Administration and George Soros (through the SPLC) have teamed up again to try to convince America's cops and law enforcers that conservative American activists are potential cop killers just because of a small group of psycho criminals," the New Hampshire Tea Party Coalition said in 2010.

While it's difficult to calculate the number of sovereign believers in the US, the Southern Poverty Law Center estimates that there could be as many as 300,000.

"The sheer number of people that belong to the movement is probably the biggest concern," Johnson said. "The fact that you have people across America that believe that the federal government is an illegal entity, that a lot of state governments are illegal, and that the laws do not apply to them is very subversive to our rule of law and to our society."

Mark Potok, a senior fellow at the SPLC, told VICE News that a complete lack of racist and anti-Semitic rhetoric separates sovereign citizens from their Posse Comitatus predecessors, who distinguished their "organic citizenship" from "14th Amendment citizens," implying that black people have limited rights.

"What has happened that is very bizarre is that very large numbers of black Americans have adopted the sovereign citizens ideology, but without the racist twist," Potok said. "I would say that if you looked at sovereign citizens today, almost none of them know the racist and anti-Semitic origins of the history. You just don't hear about it anymore."

Mark Pitcavage, the director of investigative research for the Anti-Defamation League, told VICE News that the START report signals that law enforcement personnel are finally realizing the extent of the menace posed by sovereign extremists.

"The sovereign citizens movement, which has been around for almost 45 years, has had, until recent years, an uncanny ability to fly under the radar screen, even though it has caused enormous issues from scams and fraud, to paper terrorism… to actual issues of terrorism and violence," he said, noting that the Jerry and Joseph Kane shooting and the others that followed have served to underscore the threat.

"It's true that the vast majority of people who call themselves sovereign citizens are never going to get involved in a shootout with police officers," Potok conceded. "But they're still there, and should be taken very seriously."

Follow Maxwell Barna on Twitter: @MaxwellBarna