Viewing the nation's safety more broadly, he said the US is not safer than before Sept 11, 2001. "There are many more threats, and some of today's violent extremists live in the United States and claim to be American 'patriots'," Dunham said.

Non-terrorism safety threats such as climate change and pandemics are much more daunting than they were in 2001, he added.

Andrew Bacevich, a professor emeritus of history and international relations at Boston University, said far more important threats than terrorism have emerged since 9/11.

"Most specifically, of course, is the climate chaos, which affects our country and the rest of the world," Bacevich said in a podcast last week.

Chas Freeman, assistant US secretary of defense for international security affairs from 1993-94, said in an email, "The external threats to the United States have been reduced, but internal threats have escalated as the constitutional norms of governance and the rule of law have been eroded by policies and practices driven by fear and justified by expedience."

He said such erosion "has made decadence and division the major dangers to the United States".