They don't even have to be particularly careful about hiding it. People naturally won't believe it to be the case. They will desperately cling to the notion that it's just "immigrants and Muslims raaargh they hate our Western way of life because Allah!". It's the easy option.
People naturally WANT to believe that terrorist attacks and organisations are blotches of darkness and hatred on a world that is light and good. They can't handle the truth that terrorism is just a tiny glimpse, a shark fin peeking through the waves, a symptom of a darkness that runs deeper than they can bare to imagine.
Like I said - duck and cover. This **** is scripted. You just have to hope that you're not a
Redshirt like Charlie.