Homeland Security Secretary Warns of New Terror Environment Post-9/11

— -- Fifteen years after the 9/11 attacks, U.S. Secretary of Homeland Security Jeh Johnson said the U.S. is in a "new environment," with the nation's greatest risk from lone-wolf attacks and "self-radicalized" terrorists.

Johnson said on ABC's "This Week" that there is no credible evidence of an imminent terrorist threat to the United States -- but there are still risks.

"Just saying there's no specific credible threat doesn't tell the whole story," Johnson said in an interview with ABC's George Stephanopoulos at Ground Zero prior to a memorial ceremony to mark the 15th anniversary of the attacks. "That's why you see a lot of security out here today in New York City and in other places where we're observing 9/11."

Johnson said the country is safer against organized, 9/11-style terrorist attacks directed from overseas. Law enforcement and the intelligence community have become "pretty good at connecting the dots" to counter those types of threats, he said.

But he said the prevalence of social media allows ISIS to reach lone wolves and "self-radicalized" actors in the U.S. who are capable of deadly attacks in their own right.

"It makes for a more complicated homeland security environment," Johnson said.

Asked about Donald Trump's proposal to begin "extreme vetting" of refugees seeking to enter the U.S., Johnson said that country already has "pretty intense" screening of people from "particular parts of the world" and who "meet certain parameters" before they are allowed into the country.

The homeland security secretary said that over the last year, his department has added security checks and other new mechanisms for vetting refugees -- including men, women and children fleeing violence from Syria. The screening process is "multi-layered" and takes about 18 to 24 months for each person, he said, adding that the department constantly evaluates whether even more vetting is necessary "given the current threat environment."

"We’re determined to root out violent extremism and that’s what we look for," Johnson said.

Johnson also addressed recent hacks into U.S. computer systems, including state election systems. "The message I’ve been sending to state and local election officials is that there’s a range of cyber-actors out there that are pretty sophisticated right now, not just nation-states but criminal actors, hactivists, and we have to do our best to protect ... our election systems."