The nation’s top intelligence official warned this week of the threat posed by “homegrown terrorism,” though he said there was no evidence yet of an organized terrorist network operating in the U.S.

A small number of American Muslims are engaged in extremist activities at home and abroad, said Director of National Intelligence Dennis Blair Tuesday at a Senate hearing. Their focus remains largely overseas, he said, but the threat to the homeland from Americans with links to radicals abroad remains troubling.

“We are concerned that the influence of inspirational figures such as Anwar al-Awlaqi will increasingly motivate individuals toward violent extremism,” Blair said. Al-Awlaqi is the radical Yemeni cleric linked to Maj. Nidal Malik Hasan, the alleged Fort Hood shooter, as well possibly to Umar Farouk Abdulmutallab, the Nigerian who attempted to blow up an American airliner in Detroit on Christmas Day.

Blair also separately warned that al-Qaida or its affiliates overseas was planning a big attack against the U.S. in the next six months.

Blair’s concerns about homegrown terrorism points up the difficulty U.S. intelligence and law enforcement agencies have in spotting terrorists who are American citizens — the so-called “terrorist next door” phenomenon.

“The motivations for such individuals are complex and driven by a combination of personal circumstances and external factors,” Blair said.

These include feelings of alienation, concerns over American foreign policy, and ties to extremist Islamic groups and “negatively inspirational ideologues.”

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