Kansas City native Gavin Eugene Long, who died on his 29th birthday on Sunday after ambushing and killing three Baton Rouge police officers, said in online postings that he didn't want to be affiliated with any group.

Long was, however, a member of a group involved in the sovereign citizen movement.

Since 2011, the FBI has considered sovereign citizens "a growing domestic threat to law enforcement." In a bulletin, the agency wrote that they consider "sovereign-citizen extremists as comprising a domestic terrorist movement."

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Simply put, sovereign citizens believe themselves to be above the law of the land. Their reasons vary, but they don't believe they have to do things like pay taxes or respect law enforcement officials, because in their minds all governments are operating illegally.

The movement's most high-profile member to date has been Terry Nichols, the accomplice in the 1995 Oklahoma City bombing.

According to the Kansas City Star, Baton Rouge shooter Long "declared himself a sovereign in records filed with the Jackson County recorder of deeds last year."

Specifically, Long said he was a member of the Washitaw Nation of Mu'urs. J.J. MacNab, a fellow at George Washington University's Center for Cyber and Homeland Security, told the Star that the group believe themselves to be native of the North American continent and therefore about the laws of any country, state, or city.

The Baton Rouge shooter's attempt to affiliate himself with them shows "Long's attempt to separate his flesh and blood 'indigenous' self from his legal entity self," MacNab said.

A video, which Long said he was recording from Dallas after the police killings there, condemns any peaceful protest.

"It's only fighting back or money, that's all they care about -- revenue and blood," Long said in the video.

His last tweet, posted about seven hours before the shooting, proclaimed: "Just [because] you shed your physical body doesn't mean you're dead."

Louisiana has dealt with deadly threats to police from those affiliated with the sovereign citizen movement previously. In 2012, some sovereign citizens were among the seven people arrested in a fatal shootout with Louisiana deputies near New Orleans.

In a 2012 profile of sovereign citizens on "60 Minutes," officials estimated there were as many as 300,000 sovereign citizens, and that the movement has been around for several decades.

That story focused on sovereign citizen Jerry Kane, a divorced, out-of-work truck driver from Ohio, who, together with his son Joseph Kane died after killing two cops in Arkansas in 2010.

Sgt. Brandon Paudert was one of the police officers killed, and his dad, former West Memphis, Arkansas, Police Chief Bob Paudert, became obsessed with understanding the movement afterwards.

Paudert told "60 Minutes": "They're willing to die for what they believe in. These international terrorists that bombed the twin towers, they were willing to die for their beliefs. The sovereign citizens, the Kanes, are the exact same thing."

Alfred Adask, who is considered a sovereign "guru," one of the movement's leading voices, told "60 Minutes" that to trust the government is "un-American."

"I think the government has gone far beyond its Constitutional limits," Adask said. "They think, 'Hey, we're the government. We can do anything.' And some people are saying, 'No, I don't think you can.'"

Adask is among the many sovereign citizens who have advocated violence on behalf of their cause. He said striking fear in his opponents, whomever they are, is necessary.

"You know, I find it troubling that the government would try to restrict our right to keep and bear arms. The threat of violence is required because they will not listen," Adask said. "The system will not listen to people like me unless there are other people that back me up who have guns."