Both agencies noted that in recent years there has been heightened communication with local law enforcement to help identify domestic-based threats. The official added that although the domestic groups have been relatively quiet since the 1995 bombing, the FBI has hundreds of ongoing probes involving extremist groups nationwide.
Lewis cautioned that the threat of "eco-terrorists" cannot be minimized simply because there have been no fatalities in their attacks. "When you're burning homes, buildings and ski slopes, it's just a matter of time," he said. "In my view, they have just been lucky."
Darlene Dohi, right, hugs daughter Dina Abulon at the Oklahoma City National Memorial. Dohi's husband, Peter Avillanoza, was killed in the bombing.
(Bill Waugh -- Oklahoma City Oklahoman Via AP)
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Experts attribute the weakened state of most hate groups to the death of prominent leaders in the extremist movements that left a power vacuum and dwindling membership because of infighting. Others, they say, simply distanced themselves after the Oklahoma City bombing. "They didn't sign up to kill babies," said Mark Pitcavage, the national director of fact-finding for the Anti-Defamation League.
The current void in leadership on the radical right plays a major role in assessing the immediate threat of such activists, said academics and terrorism experts.
One of the most significant losses for anti-government zealots was the 2002 death of National Alliance founder William Pierce. Pierce wrote "The Turner Diaries," considered McVeigh's blueprint for the Oklahoma City bombing, and has received virtual cult status among far-right extremists. Last fall, Aryan Nations founder Richard G. Butler died, dividing the once formidable group into two factions, hampered by lawsuits and arrests.
The conviction of white supremacist Matthew Hale in Chicago for threatening a federal judge gutted his World Church of the Creator, which advocated the premise that "white people are the creators of all worthwhile culture and civilization." And Robert Millar, head of Elohim City, a white-separatist compound in northeastern Oklahoma linked to McVeigh, died in 2001.
"The few leaders they have left can barely drag their oxygen tanks to the meetings," said Joe Roy, chief intelligence analyst for the Southern Poverty Law Center.
Consequently, there has been no one strong voice articulating a cause, which leaves angry but aimless dissidents. "It takes someone to preach the gospel," said Robert Heibel, executive director of Mercyhurst College Institute for Intelligence Studies and a former FBI chief of counterintelligence.
Others argue that the most dangerous times can be during a power vacuum. "You have more marginal people trying to act out and hard-core believers trying to fill the void," Toole said, adding: "Everyone has to understand that they are just regrouping -- a new generation will come in."
And maybe some of the old voices will bridge the gap. After notorious former Ku Klux Klan leader David Duke was released from prison last spring, where he had served more than a year for fraud, 300 people turned out to hear him speak in New Orleans on Memorial Day -- and 67,000 tuned in through the Internet. "It just shows you just how hungry they are," Roy said.
Staff writer John Mintz and research editor Lucy Shackelford contributed to this report.