The United States has made great strides in toppling major organizations that pose a threat to national security, said Stephen E. Flynn, the founding co-director of Northeastern’s George J. Kostas Research Insti tute for Home land Security. He delivered his testimony on Wednesday to the Senate’s Committee on Home land Security and Government Affairs.

But he struck a note of caution: The nation, he said, now more than a decade removed from 9/11, continues to remain at risk from small-scale attacks from home grown terrorists and those loosely affiliated with al Qaeda.