"It requires a role for the public to play, too. Public awareness, public vigilance. 'If you see something, say something' is more than a slogan. It requires building bridges to American Muslim communities so we can help them help us," he told CNN's Chris Cuomo on "New Day."
Omar Mateen, a 29-year-old security guard from Fort Pierce, Florida, had been on a terror watch list and was looked into twice before he killed 49 people and injured 53 Sunday at a LGBT nightclub in Orlando.
Johnson said very often people close to terrorists are aware of suspicious activity.
"When you're dealing with home-grown, bound extremism, the so-called lone wolf, lone actor, it is the case that almost always somebody close to that person saw the signs, somebody close to that person was aware of the gun purchase, saw suspicious behavior which is why our efforts to build bridges to various communities around this country are so important," he said.
Johnson said it does not appear that Mateen was operating under the direction of a terrorism organization.
"At this point, it looks as if the gunman acted alone, that he was terrorist inspired, that he was not directed from a terrorist organization overseas," he said.
Johnson also pushed back on Republican presumptive presidential nominee Donald Trump's plans to ban Muslims from immigrating to the United States, calling the proposal "overly simplistic" on ABC's "Good Morning America."
"I won't comment on what the candidates say running for president. I will say that overly simplistic suggestions that we ban people from entering this country based on religion or ban people from an entire region of the world is counter-productive. It will not work," he said. "And we need to build bridges to communities, to American Muslim communities, right now to encourage them to help us in our homeland security efforts."
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