Much of the public discourse in Washington in recent weeks has focused on
possible new threats to the U.S. homeland from the Islamic State and other extremist groups.

But Homeland Security Secretary Jeh Johnson says he is even more concerned about a danger that has been around for years, since even before the Sept. 11, 2001 attacks — homegrown terrorists, or so-called “lone wolfs.’’

“In many respects this is the hardest terrorist threat to detect, and the one I worry most about,’’ Johnson said Wednesday in a speech before a business group in Ottawa, Canada, where he is visiting for talks with Canadian officials about counterterrorism, trade and other issues.

Department of Homeland Security Secretary Jeh Johnson testifies before the House Homeland Security Full Committee during a hearing on Capitol Hill in Washington on Sept. 17. (AP/Manuel Balce Ceneta)

The definition of a lone wolf has varied with time, as counterterrorism priorities have shifted. For years, the FBI maintained that the prototypical lone wolf was Timothy McVeigh, who was executed in 2001 for the Oklahoma City bombing that killed 168 people and who was associated with white supremacist groups. Others point to Aaron Alexis, who shot and killed 12 people at the Washington Navy Yard and was battling mental health issues.

Johnson, for his part, cited the 2013 Boston Marathon bombing that killed three people and injured about 260. “We worry about the potential domestic-based, home-grown terrorist threat that may be lurking in our own society — the independent actor or “lone wolf,’’’ he said. “Those who did not train at a terrorist camp or join the ranks of a terrorist organization overseas, but who are inspired at home by a group’s social media, literature or extremist ideology.’’

“In the United States, we got an example of this type of actor last year at the Boston Marathon,’’ Johnson added.

He said DHS is working to address the threat by engaging in outreach around the United States designed to steer people away from radical groups or ideology, citing his visit last week to an Islamic culture center in Ohio.

“With the help of community organizations in a position to touch those disaffected from society and who need something or someone to believe in, belong to or worship, we are stressing that violence, terrorism and groups such as ISIL are not the answer,’’ Johnson said.

And the secretary by no means left the Islamic State, also known as ISIS or ISIL, off his list of potential threats, spending part of his speech condemning the group and its violent tactics.

“We stress that, despite its slick public media and its self-proclamation to be the Islamic State of Iraq and the Levant, ISIL is neither ‘Islamic’ nor is it a ‘State,'” he said. “Contrary to the misguided belief of some, ISIL is not defending Islam, and it is not defending innocent Muslims…There is no religion, including Islam, and there is no God, including Allah, that would condone ISIL’s violent tactics.’’

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