Who are most likely U.S. domestic terrorists?

The domestic terror attack that left 76 dead in Norway was a reminder that terrorism is not the sole province of Muslim extremists.

The Norway killer drew inspiration from the Unabomber, who sent deadly mail bombs in the United States.

CBS News correspondent Bob Orr reports that the attack in Norway underscores a threat U.S. officials have been worrying about since Timothy McVeigh and Terry Nichols bombed an Oklahoma City federal building.

Domestic terrorists present a lethal and often invisible danger.

The most dangerous is the "Lone Wolf," who evades detection as he maps his plot. Law enforcement knew little about James von Brunn's white supremacist writings until after he shot up the Holocaust Museum and killed a security guard in 2009.

"Unabomber" Ted Kaczynski lived anonymously for years in a Montana cabin as he mailed out bombs and his leftist manifesto.

Domestic radical groups also span the political spectrum. Three years ago, environmental terrorists torched an upscale Seattle neighborhood.

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In 2010, nine members of Michigan's Hutaree militia were charged with conspiring to kill cops. So-called right-wing militants outnumber those on the left.

However, 10 years after 9/11, the greatest homegrown threat still comes from Islamic extremists, trained or inspired by al Qaeda-linked terrorists.

Nidal Hasan, inspired by Anwar al Awlaki, killed 13 soldiers at Fort Hood. Najibullah Zazi, schooled by al Qaeda bomb-makers, plotted to blow up New York's subways. And Faisal Shahzad, aided by the Pakistani Taliban, drove an SUV bomb into the heart of Times Square.

In all, more than four dozen U.S. citizens have been charged in various Jihadist plots in the past decade.

Terrorism knows no particular brand, knows no particular religious ideological baseline. Terrorism is a tactic that is used by all sorts of groups of all political and religious faiths and stripes.

Some critics say, in light of Norway, the U.S. should focus on right wing radicals, but Homeland Security officials say they're meeting the threat where it is. For now, the emphasis remains on al Qaeda.