"We have not only the persistent threat of foreign terrorism -- that, of course, created the Department of Homeland Security -- but we have adverse nation-states, and for the past 10 years, we've seen a significant increase in what we term 'homegrown violent extremism,'" Mayorkas told anchor George Stephanopoulos. "It is a very difficult threat landscape, and it is why that we as a community -- not just the federal government, but state and local officials and residents -- need to be alert to it and take the precautions necessary to avoid violence from occurring."