(CNN) —  

Secretary of Homeland Security Jeh Johnson said the attack on a French publication Wednesday that killed 12 people underscores the increasingly complex threat the world faces.

More terrorist groups are popping up around the world and more people are joining terrorist organizations, Johnson told reporters on the Hill on Wednesday. He also highlighted the uptick in “lone wolf actors” who may be inspired by known terrorist groups but act on their own. Johnson added that investigators are still trying to determine the exact “nature” of the attack.

“[These are] actors who may lurk within our society, that could strike with little notice, commit an act of violence because they have been inspired by things they have seen on the internet, social media, in literature, without accepting a direct order…from a terrorist organization,” Johnson said.

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And countering what he called the “evolving nature of the terrorist threat” calls for more collaboration between federal officials and state and local law enforcement, Johnson said.

It’s also “important that the public itself be involved in our homeland security,” he said.

Johnson would not say whether he is considering raising the terror threat level in the U.S.

“We continually put out bulletins to state and local law enforcement,” Johnson said. “That’s something we do on a routine basis. And I’m sure we’ll be doing more of that in the short term in the coming days.

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