Charles Marino, a former senior law enforcement adviser at the Department of Homeland Security under President Obama, spoke Monday on NewsNation about migrant inflows in Europe and the United States. He said those inflows have brought people who may cause harm as well as ideas that can radicalize people already living in those places, thereby enlarging the overall danger.

Marino explained that many of the arrivals were not screened, leaving officials without knowledge of who they are. He said the United States has faced a raised threat for some time, that law enforcement and intelligence services know this and watch it closely, and that adversaries such as Iran and its partners could respond in irregular ways.

Europe has also handled a large migrant influx, he noted, matching conditions seen in America under earlier administrations.