Fifteen years after the 9/11 attacks, Homeland Security Secretary Jeh Johnson said Sunday that terrorist-inspired "lone wolf" attacks remain the greatest threat to the nation's security.
"It's a new environment. It's a new phenomenon," Johnson told CNN's Jake Tapper on "State of the Union."
Johnson said the country is better at detecting 9/11-style attacks, but added, "it's more challenging with this new environment that we're in."
He was referring to the fact that ISIS has been able to reach more lone wolves through social media.
"It makes for a more complicated homeland security environment," Johnson told ABC News.
In his interview with ABC News, Johnson also addressed Donald Trump's proposal to begin "extreme vetting" of refugees who enter the country.
Johnson said the U.S. already has a "pretty intense" vetting system for people who come from "particular parts of the world."
He added that the screening process is "multi-layered" and takes 18 to 24 months for each person.